I just checked my owner's manual for Keystone, and it simply says, "Level". It does not specify say "how" level.  I do know from experience (after owning 3 different travel trailers now), the camper does not have to be "perfectly" level for the refrigerator to work.  
I think a good rule of thumb is simple:  If the camper is so uncomfortable you cannot walk in it without feeling like your on a roller coaster, or lay in bed without rolling out, or sliding to the floor, your refrigerator will probably have poor performance also.  Being "out of level" (that much) will not hurt them, they just won't cool as efficient, or may stop completely.