I respectfully disagree with your idea because:
- traps moisture (there will always be some)
- moisture = mold = rot = metal rust, etc
Same with spray foam. When plywood floor gets water in it, there must be a way for it to get out... vibration on the road may cause delamination. I would not want to lay under the AS to spray the foam... and you do not want to spray inside the belly pan....rust, etc
Part of why AS uses the pink stuff. Gets wet, lots of airspace so dries quick..
Peace and Blessings..