Was the "Foil, total enclosed underbelly" a dealer add on? The Thermal Package is
Insulated, heated and enclosed underbelly, foil insulation in roof and slideout floors
and has an MSRP of $472.
If I was a guessing man, I would guess that the $825 option is a dealer add on. They likely just put a layer of the foil under or over the enclosed underbelly and charged twice as much.