I believe spray foam will hold up fine, be easy, if expensive. I applied foam-in-a-can to my Black tank dump pipe. It's appearance won't win any awards, but 20,000 miles later it is as good as day one.
A cheaper alternative would be to use sheet foam from the lumber yard and carefully cut it to fit the various spaces underneath. You could even use foam-in-a-can to seal any gaps around the edges. I would choose foil faced sheet foam since the foil facing the air space below would add to the R value. If you wanted to get really anal you could space to foam down about ¾ inch below the floor and get the advantage of two foil surfaces facing an air gap. Spray foam on the other hand could coat the steel frame as well and add to the total R value.
My DW says just use house slippers. God love her.
John Michael Linck - Toymaker
Camping since 1960 - Scamp 13' Oak
Subaru Outback 4 cyl cvt