You just look at certain other team members who don't go and ask yourself "Is this how I want to look like in 10 years?".

But few things help me out in during my Monday-Thursday travel schedules.
Signed up for a chain gym (LA Fitness) which is luckily available in both my base and all sites I have been in my short career.
Have a coworker in the same chain, and we share car, etc. So both are accountable for each other. No one wants to let the other person down.
Most times, if you get out by a reasonable amount (7PMish), you can just go straight to the gym and then go back to hotel. Unless you have a load to do when you go back or likes to work out for 3 hours, 1 extra hour won't destroy your night.

Not a big fan of hotel gym. Most are limited to some treadmills and pulleys. If you are out in the middle of nowhere, kind of out of luck. But you should also check if the hotel you are staying in has some kind of arrangement with the local gym. Where I am right now, I think the Townplace Suites offer free use of nearby Planet Fitness.