Our first cat loved traveling in the motorhome. That's one of the reasons we like RVs, we can take our pet with us instead of leaving them at home or at a kennel. In the motorhome, we put the litter box in the shower and moved it out when we needed the shower. Not ideal, but it worked. We didn't keep a carrier or anything, just let her run around the RV like she owned it.
Our current cat doesn't much care for traveling. With the trailer, we do keep him in a large carrier in the back seat of the truck while on the road. We have a pass through cat door into the outside compartment for his litter box. Once we get the trailer set up, we just let him loose in the trailer. He's usually OK with being in the trailer and doesn't try to get out. In fact, when we went to get him to put him in the carrier for our trip home from Thanksgiving, he really wanted to stay in the trailer rather than get in the carrier to go home.
We ordered a large carrier for the truck, its probably 3' x 4'. Maybe too big?