Agree with the above suggestions. I'd have him in and out, feed him in it, have one of his favourite beds in there, his favourite toys, play with him in there. Maybe for travelling -- crate him (assuming you have him crate trained -- the safest way to transport a dog) and put a dark sheet over it so it is calm and a dark retreat. Many dogs that do not like travel do far far better crates *and covered*. This is how I often transported rescue dogs. Does he have a kong? If not, I'd get him one, stuff and use it regularly for him, then he can have a stuffed kong in his crate when it is time to travel.
Travel anxiety tends to come because the dog has not been acclimatised to the vehicle. This takes time. EG get him used to it while it isn't moving, after a week or two, drive it just around the block, etc.
You can use a few drops of rescue remedy -- some find this works.
Also turn on the radio to some calm music -- both when the RV is just parked, and always when moving, Classical music is good!
The music serves to calm but also acts as a kind of 'white noise' to get rid of startling noises that may make him nervous.
You can also buy a dog hormone (DAP) spray that is very calming. I would use all these before I'd consider medications. Also -- give him a good long walk/play/obedience practice session before getting in the RV for travel. A tired dog is a sleeping dog that isn't an anxious dog...