I don't agree with 30°. If we are talking an AC/DC (compressor driven) frig then 30° may be okay but if it is an absorption frig 3° (maybe 6° on newer models) is the maximum.
"Whether you are in a trailer, a full-scale RV, or anything in between, your refrigerator should not be turned on if it is leaning more than 3 degree's from side to side or 6 degree's from front to rear to avoid unfortunate damage. If you can walk around without feeling like your RV is leaning (in other words, if you have the general feel that it is level) then it should be fine".
"Damage can occur in only 20 minutes if a refrigerator is left running parked on a steep slope. If you are just traveling and going into a restaurant or the store, for instance, and you believe you are not level, then simply turn the refrigerator off and just remember to turn it back on when you return. Being level is not an issue when you are driving on the road, even in the mountains".