From what I have seen the nut is NOT welded. and I don't think there is room 'tewwn the tank and floor to work. You could drop the lower wrap at the tank, it is hinged at the bottom, and see. To do that you need to remove the screws at the top, just below the rub rail.
The chairs were mounted on a pedestal that was bolted to the floor. they were not designed for seating while in motion. If you can find chairs like they install in newer MH you could bolt them in with belts and have 2 more seats.
The newer style are a flexsteel seat.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato