Limited time and space

Check with you health insurer. A lot of them can give you a list of gyms where you get a discount through your insurance on day passes. One of my friends on here is also a truck drive and that is what he does. You can also see what is nearby and and sign up for the "free day pass" to try out the gym...You'd have to listen to their spiel and pretend you might join, but it might be worth free.

Instead of watching a movie or reading a book, do laps around the parking lot/rest stop.
Look up some body weight exercises (squats, lunges, curls, flys, etc) that use dumbbells (I know cab storage space is tight but hopefully you can find a spot for two dumbbells) that you can do outside of the cab.
Learn some basic yoga poses (helps with the stretching/flexibility after sitting countless hours at a time). That only takes minimal equipment and space (the mat can also be used when doing the body weight exercises).
10 minutes of focused activity 3x a day can be just as good as a single 30 minute workout.
If the truck stop amenities (showers, game room, laundry, etc) are on the second floor, find a staircase instead of the elevator.
If you have a choice of parking spots, park the furthest away (that you're securely comfortable with) so you get a few more steps in.
Ask if there are nearby hiking trails.
Can you strap a bicycle to the back of your cab? I've seen several trucks on the interstate with bicycles.
Choose healthier food options for both meals and snacks.
Get a pedometer and challenge yourself to find ways to increase your steps by 5% each day.

Hope some these give you an idea or two. Good luck!