Ya, knowing if you mean 21' hitch to bumper or a 21' box makes a difference.
Would two of your kids share a bed? If so, there's the
Keystone Fireside 19 which has a double bed with bunk over top and then the couch and dinette can be made into beds. It seems to have a long hitch though, so 23'4" towing length with the 19' box.
If you mean 21' box, the
Lance 2185 has a triple bunk and queen sized bed.
The
Jayco 184BH is 20" hitch to bumper. It only has double bunks but I believe both the sofa and dinette can be made into a bed.
If I think of anything else I'll post again.