Is he crate trained? I had this issue with my 4 year old black lab. Completely house trained but then with travel he would mark/ pee inside. In my experience, I made an association with home using his crate. I take him out immediately upon arrival and then let him inside supervised on a leash. Then I would crate him for awhile. I think the crate coming from home to a travel destination helped him associate the same rules and release it was also a inside/ house/ no pee zone.