So until recently I didn't know that the aluminum fins in the back were
just clip ons to the main cooling pipe. So, now I'm pondering...

Take some aluminum, and, essentially, line the inside of the fridge.
Clip this to the cooling pipe. Add some additional fins, in places that
you won't be able to use anyway (behind and below that pipe. You might
be able to gain some room in the fridge (removing the existing fins) ,
without loosing any cooling capability. Might even gain some, as the
walls and floor would now be conducting heat directly to the cold pipe.

Oh, drats. Just remembered the shelves fit snugly into the side walls.
Okay, still aluminum floor rear and top would still be a pretty big
radiation surfaces.

The other side of the cooling coin is getting rid of the heat. I've
found that when camping, the curtains tend to block some of the heat
from escaping from the back side of the fridge. If the cook top lid is
up, that too, tends to block some of the heat escaping.