Our old cat that has to travel is put in his carrier and closed in the bathroom or bedroom. She doesn't cry for very long and snuggles down. The carrier makes her feel safe. Once we're set up and finished going in and out of the rv constantly, we let her out. She'll park herself on the sofa or bed then. You do have to be careful of going in and out if the RV because cats and dogs both can move warp speed out of the door and be lost.
If it's a long trip we don't let her out of the bathroom but will put her in the litter box during rest stops. When ready to leave back in the carrier she goes. With this routine we've never had a pet get away from us yet.