When we went to AK with our catt spike, we used whatever it was that COSTCO sold at the time. Our rig at that time had a bathtub/shower and the litter box went in there. I put an artificial turf carpet under the litter box. Spike was only 16 years old at the time and was quite happy for the ten week we traveled up and back.
He would even enter the rv on his own when we were loading rather than be home with every other day play time with our eldezt daughter.
He only whet outside when we were loading, he didn't like being alone.