Sounds kinda like my job.
See if you can get into an extended stay hotel with a mini-kitchen. This will let you do some of your own meal prep. I bring an insulated lunch box type cooler on my trips and hit the grocery for microwave lunches. Not the best, but WAY WAY better than the on-site cafeterias meals. I also bring my own snacks for the mid-day hungry. Again way better than what the vending machines will offer, and a crap-load cheaper too.
Just because you are eating out doesn't FORCE you into bad food choices. You can goto McD's and get a gut-bomb, and fries, or subway for a 6" non-mayo sub, and skip the chips.
Get the nutrition information for the resturant you are eating at, and make better choices. Without the information, its harder to know what you are choosing.
When you get to the offiice/work place take the stairs, and skip the elevator... well not if you're working on the 30th floor, but you get the idea.
Park a little farther out in the parking lot, and get an extra 1/4 mile of walking.
No one of these things will make a big change, but taken together you can set yourself up for a better result.