We have an old (95) Allegro with no slide-outs. For our sweet Jazzy (who took her last trip with us in the winter) we put the cat box under the bathroom sink. In order to make room for this we replaced the original sink with a vessel sink that, since it set on top of the new counter, left a bit more space underneath. We also took down both the cabinet door and the bathroom door. I just used a tension rod with curtain for privacy.
Jazzy was a very timid rescue cat but she took to RVing like a duck to water. We started with her when she was 12 years old. To encourage her to want to be in the RV she got special food and treats. It must have worked because she was ready to hop in every time the door opened. We got a pet stroller for her since she did not like being in the kittywalk.
Our newest family member, Gatsby, is much bigger than Jazzy was (she was 8 pounds, he is 17) so we will just have to see if he can use the litle box under the sink. If not, I may just take the door off the area under the bed and put his box there. I pretty much scoop as soon as they deposit anyway and like an earlier poster I worry about litter getting down the shower drain.
I hope you have many fun, safe trips with your furkids. ;-)