Idea is to have the tow bar level from RV to car. But that can vary on different RV and how it's loaded and tires, etc. so some basic measurements is one good way to go and it is not a terribly critical thing. Just the more off level it is, the more stress is put on the connections.
If you have the RV and car on somewhat level ground and measure from tow bar to ground when it is level, then measure the hitch from it to ground and see how much difference you get and that will be how much drop you need in the hitch!
But that is something that changes as we drive up and down over things like a sloped drive, so it is not a precise thing, just something better not to go down the road with the tow bar running uphill at 30-40 degrees!
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Richard
Why no RV year, make and floorplan on MY signature as we suggest for others?
I currently DO NOT have one!