First off welcome all of you to the forums.
Two inch will work but some added heat to keep the space warm will be needed.
I would build skirting walls from 1/4 in plywood and 2X2 lumber painted on the outside.
Using construction adhesive glue the foam to the plywood and seal the outside joints with chalking.
Big thing is to keep all the hoses from freezing.
2016 F350 6.7L LB CC Reese 28K 2014 Chaparral Lite 266sab
"Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety." 2014 19 days camping 2015 17 days camping201620 days camping