Take them in a harness in the car everywhere you go and in a carrier if necessary. Keep the harness on 24x7. It's taken all of our cats 4 weeks to settle down.
Then they loved to travel in the MH with no carrier. They always wore the harness and a 6' red leash when driving so that we could easily find them before deploying the slides. Yes they could always get out of the harness but didn't and we only used it in the RV.
Our last cat learned to hate the car because it started to represent the vet.
And no you cannot train a cat but they can train you!