I have two large dogs in my 21ft class C. One usually lays down on the passenger seat (I have a portable center console that she puts her head on) and the other is usually on the couch behind the passenger seat.
I find the hardest part is when I have company in the RV with the dogs. That's when it gets a bit crowded floor-space-wise.
My dad had a 5er with bunk beds. He removed the bottom bunk and set up crates under the top bunk. He had akitas, so the crates just fit.
If you look for an RV with bunks, you could do something similar.
Also, I have a parrot who camps with me. I have his cage set up on the dinette table and have strapped it to the table with bungie cords to keep it secure. You should consider how to keep the crates secure while traveling.
