I think I found the ideal riser for my particular tow bar non-level condition.
My E350 hitch receiver is 6" lower than level with my 2006 Jeep Liberty. Therefore I need a 6" riser. I found what I believe to be the right riser on ebay
HERE.
This is the picture in the ebay ad, but it appears to be a 4" riser.
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I have been in communication with the supplier, asking these two questions with his answers.
Ron's Question #1: Will your 6" riser reduce the 5000 pound rating of the hitch?
Supplier's Answer: I do not see why it would lessen your tow capacity. It is very strong.
Ron's Question #2: My motor home's hitch barely protrudes beyond the rear bumper. Measuring accurately, I determined I need a minimum of 2.5" from your riser's lower hitch pin hole to the back side of your upper square receiver. Does your 6" riser meet this dimensional requirement?
Supplier's Answer: It will fit on there perfect.
Not exactly the detailed answers I hoped to get, but I think I will order one of these and give it a try.