I have grown to like eating alone at restaurants, I put my headphones in, listen to a podcast or something and just cruise the internet on my phone. The trick is to not care what people think of you as you will probably never see them again anyways.
Really, it is tough living in hotels. For per diem and still eating healthy, I have adapted intermittent fasting.
As in, I don't eat breakfast, first meal is at lunch time, second meal is supper and that's it. This is nice because you save the 10$ of whatever for breakfast, but can also get away with a little more for lunch and supper. Also, I try to drink nothing but water.
For lunch, usually I just get Subway or something. I really dgaf about how much fat I eat, so I will get a salad and extra chicken for lunch and some fatty dressing. If I get a sub I will just get a 6" with extra meat. A lot of the places I end up, the options are basically subway. When I can I will get groceries and eat that, but if you don't get a fridge you're basically uckfayed. +1 for kitchenette but if that isn't an option then that sucks.
If you really want poverty spec, get some bananas, some fajitas, and some peanut butter and mix it all together and just have those. No need for a fridge, pretty dense meal, cheap.