If it still operates, getting it level before you turn it on is all that's needed. Actually, as long as it's still working, hauling it around and sloshing things a bit is considered a safe practice. The damage only happens when it's operating, sitting still, and unlevel all at the same time; and it's seldom an "instant failure" - just cumulative degradation. It does seem like once you notice one isn't working very well it usually isn't long before it doesn't work at all.
And it seems that the smaller ones, found in little trailers etc., are more likely to be unlevel and more likely to be damaged by being unlevel than the bigger refrigerators found in BIG campers (that would tip over if they're not kept level). [The taller the 'fridge is, the better the gravity works on it.]