Bast,
Have you considered looking at a toy hauler? If you are alone and in the wheelchair, I see why you would want a lift...but if the person in the chair has a helper to drop the ramp...simple is better and more reliable.
They even make them with power drop and retract ramps I think. The difference is...if the electric quits on the ramp, you can still get it down manually. The lift does not work so easy in emergency/manual mode. (Some do not even have manual mode now.)
The other thing you might want to look at (don't laugh now) is a horse trailer with living quarters in the front.
These are RUGGED monsters with heavy duty axles and floors in them. Much more durable than any standard RV I have seen.
Making the open horse area the way you want it is very easy and a fold-down ramp would be super easy to add to one of the swing-door models. Either one of these would be a lot less $$ than modifying a regular trailer for a lift.
The horse trailer is all set to have a huge section usable for showering. No water gets into the front of it by design. You could cover the floor with the padded vinyl that saves bones in the event of a fall.
Showering with a center drain would allow for a helper to do their thing un-cramped.
Some of them are every bit as nice as the best TT's.
https://www.fthr.com/products/horse-trailers/living-quarters
Or add a few windows, a clever graphic...and the interior enhancements necessary for your needs...to one like these:
https://www.dhmco.com/harmar-horse-trailers/living-quarters/2-Horse/9083/
https://www.dhmco.com/cross-trailers-rvs/7766/
Not sure what you going to pay for that 36...but when you design exactly what you need, you can often do more with less.
An awning/screen for the rear and that big ramp can be a wonderful situation while boondocking or in a park.
I would talk to the horse trailer manufacturers. Tell them what you need and see how they could mod a stock design right from the factory. Expanding their customer base would be a good thing.
There are also members right here that could re-purpose one to your specifics...and do it exactly the way you wanted it. One look at the framing/fixtures on them and it is plain to see how the rugged design would be a durability/usability advantage for many years.
Best of luck to you!