Park the RV next to a wall. Climb a ladder and place the level at approximately the correct height of the highest point on the coach. This could be an antenna or whatever. With a level laser, get on the roof of the coach and hold a white board facing the laser beam. You should be within a couple inches. Simply measure up or down from the beam.
Mark the height on a decal or label and place it on the dash where the driver can easily see it.