I think a Sprinter RV (like ours with seat belts for only four), usually in the mid-20 feet range, could be too tight for a family of five. I'd probably go bigger, as you said, in the 30' range.
(Not to hijack into a side topic but you mentioned the kids anticipating sleeping late, going to the bathroom, and walking around while you are on the road. If I understood correctly that you meant while the RV is underway on the highway, you may want to guide them into re-thinking that vision. Just on a practical basis, the slides come in and most beds aren't really available then. But more important, everybody in an RV is so much safer belted in and it's easier for the driver to concentrate.)