20' or less; sleeps 6; no tent ends?

20 foot is really restrictive. I have a 20 footer, and it has a queen bed, and a dinette that turns into a single. I believe there's also a 20 foot floor plan with racks instead of the dinette... problem is, racks aren't strong enough for adults and I'd barely trust them to sleep an older child.

The "kitchen" floor in mine is large enough that a single size sleeping mat / air bed could lay there.

I'm sure someone with more experience will have a better suggestion, but I don't think 20ft is enough for all that you're looking for.