Roadlotus, (great name!)
First, I'm sorry for your loss.
As there are many threads on TV selections - my comments are towards the used preowned car market. While I am not familiar with your area, in general - across the nation pre-owned vehicles are fetching insane prices - in some instances near the price of a new vehicle. For my money, I would not rule out new - if one is available. Not many if any in my area.
You say your downsizing to a TT - is that a full time -tour around, or are you going to stay put in a smaller circle of location? I think that makes a difference in your TV choice. If you intend to travel - payload and towing are really important. As for fitting in the garage - your leaving that phase, I would not worry here.
The Rivan is a very risky choice - in my opinion- unproven manufacturer, unproven promised performance, and traveling around would be a pain looking for charging all the time. But - that's just my take.
Diesel is the way to - but fuel prices are up, but then what isn't. A stout, well equipped 1/2 ton has proven to be a good choice for many - and the WDH certainly helps here - although I hated using mine - so I went to 2500 GMC. At $31K I don't think there will be much worth the asking price - but you have time to be selective. I wish you well in your search.
RB