I've always leveled using the floor of the passthrough as the reference. The floor is the first thing screwed down to the frame when in enters the factory door and is continuous from from to back.
I use a digital level on the floor, get it zeroed and then screw a 'Never Fade' level
to the outside wall just forward of the passthrough hatch. On my previous TT, it involved some back and forth from the manual A-frame jack to the level (until I memorized how many revs equaled one degree). On my new 5er, I made sure I can see the level while operating the landing gear extend/retract switch.
Never had a problem with fridge operation.
If you've ever seen a picture of the back of an absorption fridge
, the basic requirement is that the working fluid be able to run back to the bottom through all that zig-zag absorber tubing under gravity alone. Therefore the tubing has to be all 'downhill' - i.e. not so out of level that fluid could pool and form a blockage anywhere on the way down.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam