Verified answer

JD is right on the money, but it's also good to put down a board or two on the low side of your camping site. That way you don't have to jack it up so far. There are plastic stacking blocks that you can buy to do the same thing, but I just use old 1X8's to park on. I also have a cheap level that I keep in the camper that I use when levelling it up. If you don't have one, just fill a 2 liter coke bottle almost all the way to the top, leaving a small bubble of air in it. Lay it on the floor and use it as a redneck level. It's not pretty, but it works.

For a conventional camper you can buy adjustable corner support jacks made just for this purpose. They can be purchased at any trailer supply store or trailer dealership. If your trailer isn't that old, it should already have drop down corner supports that crank down to level your camper while parked, already mounted on each corner.

use the lever

put board under the tires and balance the camper out...use the jack to also balance the camper

always try to find a flat spot which would make easier

Jacks do work best, but i would go with the blocks, just watch out, they have a tendency to harvest lots of earwigs.