Just as you have to take time out of your day to dedicate to working out you have to do the same while on the road.
It can be a bit more difficult while out on a trip. Sometimes just sheer fatigue can be a real problem. There are times when I am really tired and I will NOT work out even though I have the strong overwhelming compulsion to do so.
Being out on the road can be stressful at times, both emotionally and physically, (I just got hom from a 36 day trip, and the phone is ringing off the hook for me to go back out on my off time). I will workout at every opportunity that I can get, (other than the complete exhaustion situation).
As some of the other people have stated a number of contracts/work agreements will state the requirement/desire of being booked in hotels with workout facilities. I have been to some hotels that have excellent facilities, one actually doubles as a public health club. Some hotels will not be so well equipped or the equipment is old or broken. There is quite a demand for workout facilties so many hotels are installing or upgrading .
For those that don't have anything they might have opportunites available with local gyms for either free or a small charge. The only problem with these are getting to the location. You might be able to get the hotel courtesy van to take you there, but as they mostly pickup guests at airports you might get stuck at the gym for a while waiting for a lift. I have yet to call a cab for transport mainly because of the expense (I don't make those major airline megabucks!). If there are absolutely nothing available I will weather permitting either run, (not my preferred workout , too hard on my shins) or I will do calisthenics in my hotel room, or go for loooong walks or a combination of all of the above.
Overall the effort is worth it. There is a fair amount of crewmembers that do workout, so you will have plenty of company.