Ed, those HF kit trailers are 4' wide, right? If you build the deck wider with wheel wells and skirts then you could increase the beam to match the tow vehicle (somewhere between 5 and 6 feet). Without any elbow room on the sides of the mattress you'd probably have to crawl over it. If offset to one side there might be enough isle to get past it. Think about where you want the door (assuming only one) how big it can/must be. That will constrain body geometry. Then I'd use the template to maximize interior space with least aero penalty.
More food for thought:
1. Airstream Basecamp
2. Frankenhauler:
web page forum thread
3. Tiny toy hauler (Scamp hack):
blog post