[Diesel-Gypsy]

Pat, I think many of us Suites owners have used canned spray foam to fill some of the "built-in" gaps between the frame & underbelly coroplast material.
That will help seal up the underneath some, but it is still not really well sealed.
The idea of using foam-board insulation is because there are / will be times that servicing under there requires access.
Spray foam would possibly hinder the access required to service those slide rams, and plumbing as well as wiring & ducting.

Pat have you taken the time to drop your under-belly cover after driving in the rain...?

RL