Try to gain weight during travel and when you are back burn the calories. Simple technique is that like some actors do it like run in the morning if gym is not there.
Keep a track of your fitness when you're not travelling. If you are not into heavy workouts you can just do walking and yoga regularly. If you maintain your fitness properly, you can enjoy thoroughly when you are travelling without worrying about it.
What prevents you from travelling and exercising for 15 minutes a day occasionally? That's easy. As a trainer, I can tell you that fitness doesn't have to be done in the gym. You can travel to different countries and do exercises wherever you stay. I always have my smart watch, smart scale https://www.vont.com/product/smart-scale-bathroom-scale-weight-scale/ and a few apps (yes, on my smartphone) with me and it all helps me keep track of my health. I love to eat good food and I always try the local food of the country I'm visiting. Sometimes I skip sports, but I make up for it by walking and walking around the cities.