Yeah, there is a lot that can be done with enclosing the under belly. Is it Forum member LarryJM (?) that has the pictures of his work in that area? He really did a great job planning out and installing everything, leaving some panels for access points in case repair was needed later. Really, common sense will get a lot of the design done.
About the foam board: This type of insulation is really a godsend for the DIY'er. It cuts easily and is light, doesn't hardly absorb moisture, and, if installed right, puts a lot of insulation value into a small space. I have it installed throughout my old Starcraft and I think making sure there are no gaps for air convection is one secret. FWIW, if I close all the windows in the morning on a 90f day, the trailer will only heat to 75f by noon or so. The same works for heating. One 1500W space heater is more than enough to heat at below freezing.