For my 14 year old part-Maine Coon house cat, the litter box in the shower works fine. But it's a non-standard litter box, a plastic storage box from s'Lowe's. Smaller footprint, higher sides, better 'handles' than ones from the pet store. Works fine in my unusually small wet bath; litter doesn't get kicked out, and it's easily removed when I use the toilet or shower.
I use clumping litter. Just remember to bag it, not flush it.
I reduce odor by giving Miz Plush only non-fish food, and no milk.
To get the cats accustomed to RV travel, sit with them in it (parked) for an hour. Next day, two hours. At the end of each, give them a major treat, or their daily feeding. Next day, drive them around the block, then the treat or food. Next day 1/2 hour drive, treat or food. See how it goes. From the start, find a gentle way to teach them that the driver's lap & footwell are off-limits; I used a water pistol and 'No!'.
Miz Plush is a very timid cat despite being a husky-built 14 pounder. She never 'rushes' the door. I keep her in a harness and leash for safety when when carrying her out and back in from the house. It paid off when I took a hard pratfall on the icy steps one morning last winter.
I keep a soft-sided pet carrier folded up under the couch, for use in RV emergencies. I have a short leash tied off in the overcab bunk so I can slip her harness on and restrain her during things like ferry inspections.
Even after 8 years, she still get her treat (canned food) sometime during any trip. Her purr is louder & longer at night in the camper than at home, so I think she likes it. We've traveled for as much as 3 weeks.
Hope this helps. Happy travels!
Jim, "Mo' coffee!"
'06 Tiger CX 'C Minus' on a Silverado 2500HD 4x4, 8.1 & Allison (aka 'Loafer's Glory')