If you're talking 21' box, the model I have may work for you.
Jayco X213 It's got double bunks at the front, and then a king size hard slide bed at the back. The bed is huge. We've got 3 kids and the littlest just sleeps with us on the king. As she gets bigger, we can just flip down the sofa in a couple seconds and she can sleep on it. It's got a 24' towing length.
I can't seem to find the
Jayco 228 for 2015, but that's the link to the 2014 and if you can find one on a dealer's lot, it has triple bunks and a queen bed. It's 25'11" towing length though and a 22' box so likely bigger than you want.