http://www.rhhfoamsystems.com/
These guys make a spray foam that comes in a disposable container kit with several nozzles. (nozzles are one time use) This foam is similar to the "Great Stuff" foam available at Maynards or Home Depot but in a much more cost effective amount. Do the whole underside of your bus using the cans and you will spend... well I would guess well over a thousand. The problem with this foam is that once you spray it into the floor it will want to flop out as a result of gravity, unless of course you have access to a ships gimble that can roll your bus onto the roof then fill er up, obviously not feasible. One alternative that I plan,(but have not actually tried yet), is to screw or tack weld 1" X 1" wire mesh to the bottom side of the bus floor then spray the foam in between the holes to fill the floor cavity. The wire mesh would form a sort of net to hold the foam in place until it dried solid.
The actual name for this type of foam is polyicynene . Many companys have their own name for it but thats the type, just like a styrofoam cup is expanded polystyrene (EPS) and the pink or blue stuff is extruded polystyrene (XEPS). Polyicynene has an r- value of about 7 per inch as compared to expanded and extruded at 3.5 and 5 respectively. You get what you pay for in this case. Another plus is that the polyicynene acts as a vapour barrier as well, meaning a very effective rust proofing if installed properly. Be sure not to encase all your lines though or you will have a buggar of a time getting at them to fix them or worse yet, the water or other fluid gets trapped in your floor system causing even more damage. Let us know what you do...