If you significantly increase the bus's weight your going to want to change the suspension anyway. So, really it's just a question of how much of a conversion are you doing. With a bus you can just rip out the seats add a bed and a fridge and call it done as you keep the weight about the same. However if you going wild and thinking about adding your hot-tub then I think a firetruck may be a good option.
I tore so much industrial strength junk out of it. The seats? They were incredible, built to last many years of transit system abuse. Another example, a wheelchair lift that looks like it came from an aircraft carrier, and could lift an aircraft. I don't see ever re-adding that much weight. The most significant thing I've ever had was 300 gallons of potable water (and 8 companions + gear) for our last trip to BRC.
