Rod, I would love to work with SpceCraft to see if we can duplicate mine and Marsha did mention that she would built one like that, "if there was a client".
I would do bunch of that work, "pro-bono", because I can and because it would be very rewarding to see something like that again. And I think if she exhibited it at Tampa it would cause quite a stir (and orders).
There are few "fundamental considerations" of designing the flat floor unit and keep it under the 13 foot height limit.
First, because you will have two frames not one (one for floor and one for axles and basement), I-beams, C-channels or vertically stacked tubing will not work. To achieve the necessary strength these components would be either too massive, or too tall. So instead of going vertically you go horizontal. On mine it's a rectangular tubing welded to each other horizontally, not vertically. That extends 40 feet from the rear bedroom floor all the way to the front living room floor and meets up with the structural members around the pin box.
I'll divert here for the moment. There has been quite a bit of discussion about Lippert frame failures, particularly around the pin box (undersized structural steel and lousy welding). The fifth that I bought originally came with a truck which had commercial Holland hitch plate that did not articulate side to side, only forward and back.
That puts tremendous stress on the pin box when you take a corner while the truck and the fifth are not horizontal (uphill/downhill turns, etc.). Note what that pinbox and pin plate did to that Holland head.
It dug four massive divots in that steel plate (3/8" deep) around the kingpin and the edges of the pin box pin plate. This was one of the reasons I got into designing and building an ET. I did remove the fifth underbelly in that area (to service a hydraulic hose) and there was no evidence of any cracks and the welds are perfect (and substantial). If it was a Lippert frame I would expect the truck would be heading down the the road with the pinbox attached to the Holland head and the fifth heading for the ditch.
Second consideration is the air-conditioners in the basement and you already know about that. That is not as simple as it sounds. First, is the choice of the unit, not every unit is suitable for that. I have two Dometic units that were originally manufactured for the use in small permanent campers that you see sitting on the blocks in many campgrounds. Mine are no longer made, but I believe there are other models which replaced those. Third, designing the air flow and ducting is not trivial. Even though it's in the basement you need outside air to come in to extract the heat from the coils, have properly sized plenums and ducts to distribute the cold air into the fifth and return it to the unit, filter that air and be able to replace the filter and drain the condensation so it doesn't rot your basement sub-flooring.
Third consideration is the elevated position of your flat floor, it's going to be be high and you need to get to it. In this 55 footer that Marsha built.
there is a horizontal platform that comes out from just under the flat floor and it forms a "porch" (similar to what King of the Road was offering) then an angled set of stair is attached to it to climb.
On mine I have retractable stairs, hydraulically operated,
Having that little "porch" and railing is essential when climbing up there and trying to open the door with a bag of groceries. The extra step at the bottom became essential when I flipped the axles to line it it up with the Volvo, the original Dodge puller was lower and they matched the fifth to that truck.
I took those stairs out couple of times to service them (they are one of the kind),
and while I had them out I figured that it was a good opportunity to reverse engineer these and document it.
So should you fancy these, you knows who has the plans. As I said these are hydraulically actuated and Marsha does not use hydraulics, but these could be be easily re-engineered to be electrically actuated, either by a cable or a screw jack.
Lastly, you remembered my "project" to repair my flat floor frame.
This was Carriage's one and only "foray" into hydraulic "everything".
That system is massive and even the people at HWH who built it, remembered it 15 years later because it was so unique. But Carriage didn't build anything like that before or after (they stayed with electrics). One area where they screwed up was how they fed the hydraulic actuators through the frame (the stacked horizontal tubing). That required cutting holes through the tubing to install the pistons. That's done all the time in all kinds of structural members, but if you cut a hole in those you need to reinforce that area to restore it's structural integrity, they did not do that well, hence I had to repair it and "do it right".