Why no TT's that fully supports their own weight.

I have seen 5th wheel version but not travel trailer versions. I know they made them in the 50's for trailers. Stability would be the number 1 concern.

Good point on backing, no mean feat with 2 hinge points.. I drove a tractor/farm trailer for a couple of years. Learned to drive that setup backward nearly as well as forwards.. so didn't occur to me.. and that was 30 years ago so I've likely lost the skill by now ;-)

So now you have to add another $1000 for an integrated trailer dolly. Deploy the trailer dolly handle and wheels. Unhook from your TV and dolly into the campsite. TT already has batteries and the 'dolly' wouldn't have to support tongue weight, just a deploy able wheel and handle arrangement with something like a starter motor for power.

edit: or, 2 starter motors (or similar) that can hook up to each of the front wheels.. A way to stow the hitch out of the way, and then a tethered controller allowing you to control both front wheels independently. Sorta like those huge wheeled dollies they use to move buildings..

Backing up a trailer with two separate axles is very nearly impossible, at least without a tremendous amount of practice; it's the same basic physics in play as backing up a car on a tow dolly. You have two pivots in play that have to be kept lined up.

Since access to a great many campsites require one to back in a trailer, this is a very significant drawback.

I suspect there could be some rather troubling shimmy or wander problems at highway speeds, but presumably proper design and damping and such could keep them under control.

I would say handling and backing would be the biggest issues.