Not having your exact profile to work with; I had to make some assumptions and took a couple of liberties with the design. Rather than draw the frame and then put the body on, I used a HF 4x8 frame that had previously drawn and added your body to it. My renders of your idea are based on a 4x8 footprint with the walls at 4 foot also. I just put in a few basic spars to illustrate. I also framed in an opening for a roof vent, even though you didn't mention one in the description.
If, as Auntie M surmises, you will be sleeping with your head to the front (tongue end), I would recommend adding storage cabinets above your feet to add rigidity to the structure. The long hatch concerns me because you may get some flexing and/or bowing of the side walls there. Also, it would probably be fairly heavy, so you would need a good support (lifting) system.
I think it could be a workable design; it just has a few things to consider prior to starting the build.
If you would like a copy of the Sketchup file, PM me your e-mail and I'll send it. (I don't believe I can add attachments through the forum mail system.)
P.S. The people are not of my design. I imported them from the Google Sketchup library.