Here's a simple way to find front to back level' simply put a carpenters level on the vertical position of your door. When the bubble is in the middle it's level front to back.
To level side to side, simply lay the same level upright along the inside wall of your camper. You can do thar right at the door too. I use the door frame (vertical) for both directions. Floors, counter tops, stove tops are very seldom really square to the overall structure of the camper. But the door frame is always the level point, as are the walls.
