Answer

The discussion should be divided between Jewish law and theology.
 
Jewish law.
It seems to me that trust in God has no halakhic source, and therefore it should not be mentioned in the Mishneh Torah. True, in the Laws of Character Traits he mentions good and bad traits, but it seems to me that those are universal moral traits (a proper or improper psychological makeup), not religious traits such as trust in God. Trust could have been mentioned together with love of God and fear of Him (those are religious traits), but as far as I recall, such traits do not appear in the Laws of Character Traits but only in the Laws of the Foundations of the Torah. But there it is already a standard halakhic work (not like the Laws of Character Traits), and therefore trust cannot appear there either (because there is no such halakhic obligation. Love of God and fear of Him are counted commandments, and there is a Torah command regarding them).
 
Theology.
In the Guide for the Perplexed, I don’t know whether this appears, because I’m not sufficiently familiar with that work.
At the margins of the discussion I’ll add two comments:
1. The Hazon Ish, in Faith and Trust, explains that trust in God does not mean trusting that things will turn out well for us, but that what needs to happen will happen (and sometimes those are bad things for us).
2. Personally, I do not believe that the Holy One, blessed be He, is involved in our world (except perhaps in very rare cases). The world follows its natural course, and the laws of nature together with human choices direct the world. Since that is so, the question of trust does not arise at all.
Perhaps Maimonides also did not believe this, and therefore the issue of trust does not appear in his theological writings either. I don’t know. 
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Questioner:
Thank you very much. I’m familiar with the Hazon Ish. And indeed his definition is different from the accepted one in Hasidism, but he definitely still speaks of an obligation of trust according to his own meaning. 
By the way, I do not agree at all with your outlook regarding God’s presence in our lives. I read your words on your blog… 
Beyond the fact that in my opinion they plainly do not fit the words of the Sages,  I’m curious how, on a personal level, you can pray at all, and especially in times of difficulty, or in general conduct yourself before God. דווקא as for observing Jewish law and the commandments, that doesn’t seem so hard to me. This is a distinctly Leibowitzian approach. 
If you feel like it, I’d be happy to hear.
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Rabbi:
I have difficulties with the requests in prayer. The expressions of thanks are for the laws of nature that produce everything in our surroundings. The requests, in my view, are devoid of content, because the Holy One, blessed be He, is not involved except perhaps in rare cases. The only meaning I can find in requests is as an attempt to bring about such rare intervention, and therefore there is no point in doing this except in very unusual situations (when there is no natural solution), or for others who are in such situations (in the obligatory prayer that is the only meaning I find in it: to think about others).
Unlike Leibowitz, who saw this as an ideal, I see it as a problem. But at present I have no way to change it (something established by a count requires another count to permit it, and with a rabbinic enactment we require one greater in wisdom and number).
Indeed this goes against the Sages, and here one of two things must be the case: either they were mistaken (because they thought God was involved, as people used to think before they knew the laws of nature and how it operates as we do today), or the situation changed (that in their time He was more involved).
I deal with all this in my book on theology (a trilogy I am now writing). There I also explain why I do not think there is involvement of the Holy One, blessed be He, in the ongoing conduct of the world. In my opinion, beyond the fact that anyone honest will admit that we simply do not see it, the scientific picture of the world denies it.
It is indeed difficult to conduct oneself before God if one thinks this way, but on the other hand it seems to me that others also think this way, since that is what experience and common sense indicate, except that they do not dare admit it to themselves. Fooling myself does not seem like a reasonable solution. This is truly what I think, and the fact that it is difficult cannot change that.
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Yitzhak Schwartz:
And what about individual providence? And from there, to man’s purpose and path in the world? A person’s destiny and the like?
Thank you in advance
Yitzhak Schwartz, a faithful reader who always has trouble getting to the bottom of your view..
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Rabbi:
Hello Yitzhak.
My claim is that there is no individual providence (except in a passive sense: He follows what we do), but not in an active sense (that He acts in our surroundings and directs it and us). A person’s path and destiny are to live while observing the commandments given to us in the Torah. There is no need at all to assume divine involvement in order for our lives to have purpose.
By the way, I once thought that those sent back from the battle lines are the ones who built a house, married a wife, or planted a vineyard—not someone who hasn’t completed the entire Talmud or hasn’t redeemed a firstborn donkey. The point is that the purpose of life is to live, and the commandments come to tell us how to live (that is the essence of the idea of “and live by them,” and not die by them). Therefore I defined the destiny as life according to Jewish law, not the observance of Jewish law for its own sake.
I’ll say more than that: each person determines his own purpose. Jewish law is the framework within which this takes place.
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Yitzhak Schwartz:
And if providence is only in the passive sense, why is there not one single fate for everyone? Who will live, who at his end, who by water, who by fire, who healthy and who ill, who rich and who poor {isn’t it true that not everything depends only on a person’s own efforts}?
Thank you, Yitzhak
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Rabbi:
Why should there be one single fate? Each person according to his nature and environment. I didn’t understand the claim.
As for who will live, see here:
https://mikyab.net/%d7%9c%d7%9e%d7%a9%d7%9e%d7%a2%d7%95%d7%aa%d7%9d-%d7%a9%d7%9c-%d7%a8%d7%9e%d7%96%d7%99-%d7%90%d7%9c%d7%95%d7%9c-%d7%90%d7%95-%d7%90%d7%9c%d7%95%d7%9c-%d7%91%d7%9c%d7%99%d7%98%d7%90-%d7%98%d7%95/ 
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Yitzhak Schwartz:
I’ll read it, God willing. I need to reflect deeply on your positions. Thank you, thank you for your respectful replies.
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Noam:
Hello Rabbi, what about the explicit verses in the Torah that directly connect the actions of the Jewish people to divine reward? (Whether regarding rain, and in general all the blessing that He will give in the Land of Israel when we keep His commandments, long life as a result of performing certain commandments, and so on…) And all this even if I do not accept the words of the Sages, which also carry weight and must be taken into account. In addition, there is the teaching of Kabbalah, which in the simple sense connects our actions to the flow of divine abundance that we bring down into the world..
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Rabbi:
I have been asked this already dozens of times here on the site. My answer is that apparently God’s mode of conduct in the world has changed: just as miracles and prophecy disappeared, so too God’s hand disappeared from the world.
But all these theories and interpretations do not change the clear feeling that there is no such involvement in the world.