I also do not want to reinforce her feelings of mistrust. I think I am likely overthinking this based on years of conditioning while we were in a romantic relationship, whereby I am still walking on eggshells.
That's an interesting thought. What level of impact have you seen your actions/words have on her feelings of mistrust in the past, when you were together? What seemed effective? What seemed not effective? Did there seem to be a relationship between what you did or said, and how she felt?
It seems reasonable to me to inform her (which I have and would tell any friend with whom I regularly communicate) of the fact that I am traveling without providing details or allowing her to grill me about my plans.
This could be too far down the "details" track, but one thought I'm having is that there's a difference between "giving someone a heads up" about you traveling versus you "happening to mention" after the fact that you had traveled.
An example of a normal "heads up" before the fact would be to tell a family member your itinerary if you are traveling overseas, or to tell a housemate that you are hiking for 3 days and to call search & rescue if you aren't back by X time on Y day. Typically, for safety issues. Or, if you are already having coffee with a friend, and the friend asks you "so what are you looking forward to next month", to say "can't wait for my Guatemala trip, I want to see the coast and the mountains, have you ever been?" A normal focus would be on what you will do there, not on with whom you will go.
A not-normal heads up would be to tell an ex "hey, not sure if you knew this, but next month I'm going to Guatemala... just wanted to be totally honest with you".
There is a way that we tell ourselves that "being an open book" is the same as being honest, and that not being totally bared to the world with everything we're doing is "lying" somehow. I don't buy it. There is not a necessary relationship between the degree of detail I tell and my inner orientation towards truth and honesty. For example, if you asked me what I did today, I'd say "I got up, ate breakfast, went to work, and now I'm posting here between projects". Notice I didn't tell you everything, though. I didn't tell you how I got to work, what I ate for breakfast, what socks I'm wearing, when I took a breath, how many times I blinked... so, am I lying to you? It depends on what I'm trying to do, inside myself (i.e. am I actively choosing to lie by omission), and it also depends on what I know about you. Is there something that, if you knew it, you'd use against me? Are you REALLY asking me what I did today, and is that ALL you want to know? Maybe there's something about how I got to work that you really want to use against me ("I can't believe kells76 DRIVES A CAR, what a planet killer, I'm the correct person and kells76 is totally beneath me").
I have to have wisdom to understand that when someone asks me something, it's not only their literal words that are asking me, but it's their... life history, I guess, that is asking me the question, and the question can be more than the literal words that are spoken.
Understanding the question behind the words will help you understand how to answer in a way that has integrity while declining to participate in non-explicit questions.
A more extreme example (though if there is a better one, please let me know): if, back in the day, the Nazis came to your door and asked you "Are you hiding Jews in your house?" and you were, would you be lying if you said No?
You would have to have wisdom and insight to understand the question behind the literal words.
They aren't asking because they're just curious.
They are asking if you have any Jews that you will hand over to them to be murdered.
That is the real question: "Do you have any Jews in your house that we can kill?" And the correct and truthful answer to that is No, I do not have any Jews that you can kill. That is regardless of whether or not they are physically in your home.
I don't mean to use an extreme example just to fling it around, it's just that it is a very stark example of the difference between the questions we ask with our words and the questions behind the words, that we ask with our life trajectories.
Bringing it back to your situation:
I would then allow her to ask any questions she would like and be honest in my responses.
What do you think she would ask with her literal words, and what do you think the "questions behind the words" would be?