Small Trailer w/ Bunk and Enclosed/Heated Underbelly

Rockwood 2306 might be a candidate for you. Roughly 23.5 long, 3900 pounds, and full bath.

2 bunks in the back (older models have 3), and either a fixed or murphy queen bed up front.

As of this model year, the underbelly is enclosed (we just looked at one). I am not sure if it is heated, but as somebody else said, might not be too hard to extend a duct down there.