Re: spray foam under floor - who has done it?
Before I came to my senses and decided converting a skoolie was smarter than trying to make a conventional RV full time livable on a budget, I bought a 1989 cobra malibu motorhome.
Underneath this motorhome is some kind of sprayed on foam, that has black paint sprayed on top(bottom technically) of it.
This foam covered many of the surfaces poorly, trapping salty slush from the roads against the metal. It also covered several maintenance items, preventing them from being maintained without cutting foam away. The foam is crumbly in places, spongy in places, and rubbery in places and rock like in other places.
Personally, I won't be using foam underneath of my bus. The only exception *might* be some kind of rhino-liner type of coating, but even then, I'd have to think hard about it first. It would be one thing if I were tearing it down to the frame and could verify proper application everywhere. It is another thing entirely when the bus is already in one piece, and spraying foam in might make nice places for water/salt/dirt/grease/bugs/critters to hide.
If you decide to go this route, please report back about your experience with it.