Our pop up came with an icebox. You had to put lots of ice in it to keep things cold, but then there wasn't much room left to put stuff inside! On top of that it had a tendency to drip water on the floor every time you pulled something out.
After getting our camper back in shape the first real mod I did was change out that icebox for a 110V AC fridge. We got lucky and found a dorm fridge the same size so there was no cutting counters up to make it fit. However this is an electric fridge, not freon cooled so we found it doesn't get real cold. Good enough for fruits and vege's but not for meat or much else.
As you can see in the first picture above I tried to wire in an outlet. After cutting in the vent hole the thought that water could get in made me change that to hard wire the fridge. In hindsight I wish I'd had left the outlet. When I had to re-do the counter top and pull the fridge out, putting it back in had me guessing which wire went where.
There are two boards under the fridge screwed to the floor. There are pocket holes where the fridge feet sit to keep it from sliding around. I also fabricated a bracket on the back to keep it secure. All access is through the vent opening. The vent is just a plastic home vent with a slide closer on it.
Since this electric fridge didn't get very cold we've been on the hunt for a replacement. Finally found a nice freon based dorm fridge at a good price. The swap went fine, and this new fridge got colder in about an hour than the old fridge ever did. One thing I changed was to add a standard outlet instead of hard-wiring the power. The new fridge was about 1 1/2" shorter too, so I cut a piece of 1" stock to fit. It's mounted with two screws on the back side to the floor, and I used the styrofoam shipping pad underneath mounted to floor to hold it securely.
The new fridge, gap filled in. Need to finish it off with some molding.
110 AC Outlet