My bus/RV conversion weighs about 18 tons. It has 6 ton front axle and in the rear it has an 11 ton and an additional 6 or 8 ton axle. While a fire truck has a ridiculously strong suspension, if you used that for an RV, you would get a punishing amount of vibration and shaking. I guess you could ballast it, but you'll waste a lot of fuel hauling ballast around, in addition to being dangerously underpowered in some situations. I think that a fire truck is not a good candidate for an RV conversion, unless you have a very specific definition of "Recreation" in mind.
That is a great point about the suspension. The water doesn't care about bumps. A bit of searching lead me to this interesting page http://en.wikipedia.org/wiki/Fire_apparatus which supports your thesis. Since a bus is designed for riding comfort much of the suspension work is geared to that. That is something I really hadn't thought enough about.
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.
As I recall that episode didn't test 'fuel economy' as most people think of it, but instead 'fuel economy when driving straight at a constant speed on flat ground'. This was not an error -- they were up front about it; it was more a question of controlling variables. Still, it keeps us from generalizing the weight data to actual vehicle cost of operation.
