Here are the requirements:
Economical - very little build budget!
Sleeping room for 2 adults and 3 boys, the oldest being almost adult sized now.
Bathroom (portable, self contained toilet) and minimalist shower.
A cargo space for carrying small antique engines to the shows.
Light as possible to allow towing with smaller vehicles.
Teardrop shape for good aesthetics and decent aero.
I am not worried about a full-on galley and don’t want one in the back.
Since building the first one I have 3D modeled a whole bunch different designs to see how much I could fit in a small space. The design I went with was the final evolution. Here are some of the details:
It’s a fairly classic tear shape, with a 75” maximum height in the middle and 11’10” overall length.
It’s 72” wide at the front, but only 60” wide in the back (more on this later).
It’s going to be framed with 3/4” x 16ga square tube all welded together.
The front of the space will hold 3 bunk beds crosswise, one for each kid.
The back area will have benches in a U shape with a table that drops down to fill the U in to make a bed.
The adult bed will be 60” wide at the very back (head end), but will taper in at the foot due to bathroom. It’s 75” long.
A section on the left side of the will be the bathroom.
There will be a hatch on the back to access expandable cargo space under the large bed.
It will have corrugated steel for the roof, and rough cut planks for the sides to achieve a rustic look.
I was originally going to build a trailer frame from scratch out of square tubing, but then decided to look for an old boat trailer as a donor for the axle, wheels, hubs, coupler, ext. I live next to the largest lake in the southeast - should have been a good plan. But, everything started at $500 and up - which didn’t seem cheap enough to warrant the labor to convert it to what I needed. I finally found a single axle boat trailer not far away for $100 with a bad wheel bearing. I gathered up tools to fix the bearing and went to buy it, but it turned out to be a 6-lug, 5000lb axle with really big and really rusty surge brakes. The frame was also heavier than I wanted. I explained this, then he said he had another option. It was a Pace American 5’ x 8’ v-nose enclosed trailer that had been wrecked in the top front and roof. He had already stripped all the skin off, but it still had the ramp gate and was towable. It had 15” 5 lug wheels and a drop axle, all in great shape. Same price - $100.00. Sold! The trailer was almost the perfect size - though I had planned for the whole camper to be 6’ wide and this one was only 5’. I was going to widen the trailer, but then devised a plan to get around it - make it wider at the front where the width was most needed, and have it step in to the 5’ width before the wheels. Saves a lot of work, and adds some interesting design detail.
The donor trailer:
- F7007CBE-768C-4988-AE7C-F2E418044CEF.jpeg (155.91 KiB) Viewed 2305 times
The donor trailer with a tear template in place:
- 9F53E573-96B0-4B9D-A800-76D39963EBFA.jpeg (95.3 KiB) Viewed 2305 times
Is that right?
They need to be friends. One could fit three easily, or two adults and two kids topping and tailing. 
they call them Maori bunks I believe.