Used to have 4 50 and 65 pound mutts in our expansive 22', but now 3. They settle where they like on the sofa, the dinette w/ table down as daybed, and under the passenger's feet (a bit uneasy w/ noises in rig).
The big 65 lb comes to lean on passenger's lap and look out for a few minutes, then returns to dinette/bed or lays between the two cab seats on the floor (a popular spot w/ two of our departed doggies) - he moves a bit, settles for awhile, and repeat.
We don't do crates as so many do, but the fact that a dog can be content in a small box helps you to see that they do not need a big RV. Even if you had one, you know where the'll be most of the time as you trip on them.
2005 Trail Lite 213 B-Plus w/ 6.0 Chevy