Are you talking about where to put the level inside the trailer to make sure the trailer is level prior to adding attached levels to the outside to use when initially setting up the trailer when camping?
If yes, then I place a 2 foot level on the center of the floor, just inside the door, level front-to-back, then side-to-side and then attach small levels to the frame. For front-to-back, I attached the level to the front right side of the trailer frame; easy to see when adjusting the level after disconnecting from the TV.
I then placed a small level on the front of the frame for side-to-side. However, since I sometimes use an Anderson leveler, I also added a large 11" level to the top of the trailer so I can see it through the rear view mirror.
On popups, not sure how you could determine the level when camping if the levels were attached to the fridge. On my trailer, you can just see the front of the fridge from the small access door. You cannot open the door!
Trailer: Lifted 228BH, heavy duty springs and Yokohama tires
TV: 2016 GMC Sierra Z71 4x4 CC, SLT
Spare TV: Two Alaskan Malamutes
Living somewhere in ID; previously lived in Moab UT; previous to that, don't ask!