I use a carpenter's level on the floor, because the floor is the most convenient place to check on my motorhome. The level I have is far longer than it needs to be for the service, simply because I don't see any reason to replace a perfectly fine level. A 6" or 1' long one would be perfectly adequate.
Unless your fridge or whatever is wildly off-kilter compared to other things in the RV, it makes little difference what horizontal surface you use. They may not all be level with each other, but they won't be all that far off. If the eggs don't puddle in one corner of the frypan, and the shower drains properly, and your coffee isn't all slanty in the cup when you put it down on the counter, you're plenty level for the fridge.