Is your cat used to using an enclosed litter box? Some cats adapt to them, and some don't. If he is used to an enclosed box, you might try putting his old box that carries his scent in the new enclosure, or put in a little of his used litter. Also, be sure to clean the driver's seat well to remove any lingering scent of the cat urine before bringing his out there again. If he's not used to being in the RV, he might just be marking his territory. We had a cat that went on our bed our first trip out, but I think it was just a nervous reaction to the unfamiliar territory. He was fine once he got used to living in the motorhome. Cats are fussy, and sometimes it's hard to figure out what they are trying to tell you.