Rod we must have been typing at the same time, because after I posted mine I saw yours ahead of me.
I still troll the other site but not as often. Seems to have gone somewhat "subdued".
Marsha and Wyatt have lots of "basement experience" building the big 55 footers. Those are mostly flat floor throughout, or maybe a one small elevation in the front.
They said that they would love to build a copy of mine if somebody asked them but they can't afford to build one on "speculation". They think if it was exhibited they would be swamped. Few more suggestions:
I re-plumbed mine completely three years ago (to Pex tubing), took several weeks. Spent considerable time "in the cellar".
Look how much room I have there, I could actually crawl in there to do the work. This section is walled off (wall was removed to do the plumbing), it's right over the three axles and it contains two fresh water tanks, which you see here and behind them is the grey water tank and black water tank. There is a hot water radiator in the section keeping the tanks warm (with the wall up). The black water and grey water valves are also in the heated compartment (inside the walls) so those can't freeze. Because of their location this does not allow for "conventional plumbing", therefore the valves electrically operated (small DC motors) inside the compartment.
Hot water radiator because the hot water and heating system is done by two propane fired hydronic units.
Hydronic is pricey but it's an incredible alternative to the cheap ass hot air stuff. I can give you lots of pointers on it.
I needed to fix a crack in the upper frame (they did a "stupid" during manufacture but it still lasted 15 years), but you can see my welder was able to get right in the cellar.
The rig is double framed, there is a dual rectangular tube frame running from the back all the way forward to the pinbox.
Then there is the cellar space in between with vertical structural tubing members, then there is the lower frame attached to the axles.
There is one other unique aspect of that rig. The flat floor is higher in rear (typically living room, kitchen) and much lower in the front (typically up few steps into the bedroom). That put the pin box way below the typical 47 inches off the ground
As you can see here it was right down on the deck of a typical pickup. This one was not so typical since it had just a deck, no sidewalls.
It was too low for the Volvo,
so in order to go over the rails I had to flip the axles to gain another 5 inches, but I am still only at at about 42 inches which required some fancy deck design to clear things in turns.
Plus my ET sits on the bottom flanges, yours is 4 inches up. So flat floor has its "challenges", unless you pulling with commercial setup.
If you will go with anything Marsha produces I would strongly suggest that we refurbish you ET (it's one of the "seniors"). Regular Binkley does well on ET's because it's such a "gentle hitch", but a Super Binkley would give you a total peace of mind. However, replacing the front shaft mounts is highly recommended for the weights of her trailers.
Note that these now bolt on, they are matched drilled for very precise fit, the shaft pocket is extra long, therefore no need for those stupid collars. So no collars and substantially more support for the shaft. As you can see I can do the refurbishing in the field with my truck.
And I was thinking about bringing couple of refurbishment kits to the Rally.