I have a pretty limited diet - being both gluten free and dairy free can be challenging... even without adding travel... but it's becoming more 'understood' and isn't impossible. But if you also remove all fried foods from the equation? That gets SO much tougher. For fast food places? About the only places I can eat are Subway and Wendy's.
Most regular restaurant's have a couple of items on the menu that I can eat... so looking at a menu changes dramatically from being able to look at what they offer and decide what sounds good.... to becoming an exercise in finding SOMETHING I can eat without distress... or something that with a change could become edible. I've more than a few times gotten a grilled chicken sandwhich with no cheese. Then remove the bun after it arrives.
Add travel and sometimes limited options for eating? And suddenly it's not so easy. The last couple of hotels I stayed at offered complimentary breakfasts... and unless they offer fresh fruit, it usually won't work.
My solution has been to either pack a cooler (if driving) or stop at the market shortly after arriving (when flying) and making sure I have a refrigerator and microwave in the hotel room. Then I can have my almond milk for creating nutrition shakes, I can have some vegetable juice smoothies. And absolute worst case? I carry pre-made packages of tuna salad. It's not ideal and it's not my favorite meal in the world, but it does keep me from starving when I can't find anything to eat.