Ford E450 Tire Size

When I talked to the Ford engineers for my 2001 they told me those shocks are designed to handle every permutation from a blank cutaway running unbuilt down the road to a cube van with fully loaded axles. They will work in an RV, but they are far, far from optimum. The rears are likely toast by now and maybe all 4. I'd go for some designed for the weight of the rear axle (beefy) and the likely lightly or medium loaded front end (softest I could find). The air bags will probably do more for your ride with better shocks in back.

After that, stock sized tires might even give you the ride you want.

Jim