Ok there are a few reasons to want it more or less Level.. Perfect.. Well read on.
An RV Absorption Cooling Unit (Fridge) likes to be within 3 Degrees of level
What is 3 Degrees Well about 5" side to side. (that's a lot of slope)
Or 1 foot end to end for a 20' Trailer
Air conditioners condense moisture out of the air. if the Rig is "nose down" that condensate (As it is Tecnically called) may Drip down into the Living space instead of out on the roof to the ground.
And you won't be comfortable sleeping
Also if it's not "Square" the door may not open/close properly.
