The biggest benefit of spray foam is that it fills all the little cracks where most of the infiltration happens. It's virtually impossible to do that with any other form of insulation such as fiberglass or foam boards.
There are two basic kinds of spray foam - closed cell and open cell. Closed cell has a higher R-value, doesn't absorb or allow water to pass thru, cures to a hard foam and provides a lot of sound proofing. Open cell will allow water to pass thru, doesn't harden nearly as much as closed cell and doesn't provide as much sound proofing. Some have also reported that early types of open cell would shrink over time and create places for heat to infiltrate.
The early reports of spray foam emitting nastily odors like formaldehyde are pretty much gone now. There are formulas that are even considered "green".
If using spray foam, don't allow it to be exposed to sunlight as the UV will degrade it. Simple solution is to just spray paint over it.
I've used closed cell in my RV barn and open cell in my home. I'm happy with both. If I were building an RV I'd go with the closed cell for better insulation, more structural support and the sound deadening.
