Great question!
The truth is it is not easy to do all the time. The easiest to maintain a healthy diet is if you are staffed in a city/urban area with many options. MBB gives a reasonably generous food allowance and ordering healthy should not be an issue in this case.
If you are staffed in an area with limited access to healthy food - I suggest taking as many meals with you from home that you can (using a meal prep service in your city for example which is what i did a couple of times) to your client site and using the hotel microwave or client office microwave to heat these up.
In both cases the reality is working long hours affects your brain chemistry and you often want to eat unhealthy and at odd hours. I hope you manage to not succumb to these cravings!
All the best and congratulations
Udayan