When camper off truck place a level in the bed of the truck. This will help you for front to back leveling only so find a spot in a parking lot or on your property that is level. When the truck bed is level, silicone a level gauge to the driver side door of the truck. That way you can tell an exact spot to stop in a campground site when pulling in. The silicone is easy to remove off plastic door side panel later when you decide to trade in the truck.
You might be able to do the same for side to side level, but frame flexes so not sure. I usually can tell if side to side is close enough.