I have one use sheet metal such as heating and cooling shops use to cover the inside of the openings, and rivet new vent covers over them. Looks like vents but has sheet metal behind them.
Even 110 volt electric refrigerator
needs the air flow in the back so if you want any fridge
there keeping some venting might be smart. Leave a hole in the sheet metal inner panel for air flow but reduced in size.
There is this long running thread with lots of information on repairing fiberglass. Including how to patch large openings such as those if you decide to go that route. You Can Repair Fiberglass
Heck you might even want to consider getting a replacement door for that lower section. Storage accessible from outside is one thing the Scamps lack. Even if just enough space to store a few items such as a jack or extension cord it would be useful. Assuming you had a fridge
in front of it that left some room being less bulky than a 3 way LP fridge