Ron,
Why don't you just have some simple air bags installed in the rear of your PC with two filler valves (one for the left bag and one for the right bag) installed right in the bumper like I have on our pickup?
A little sagging in the rear from a heavy load in the truck ... I just take out the tire pump and put a little more pressure into the air bags. I run them with about 20 lbs. normally, but can go as high as around 100 lbs. if necessary. Very easy and straight-forward to do. Of course the ride gets better with just a little more air pressure in the bags so as to get more of the weight off the steel springs and onto some air cushion.
(I can't get my E450 Class C to sag in the rear - it actually sits a touch high in the rear regardless of my tank loads - so air bags will do me no good on reducing the somewhat harsh ride coming from the rear shocks/springs.)
