OP - When we had a 1984 24-ft rear bath Class C on E350, it wasn't older "then" than yours is "now." By then I mean when we added a leaf to the rear springs. The leaves were called "Repair Leaves" and are sold by thickness and width. Have to be cut to length, and they do NOT come with a Center Hole. It all went well except drilling for the Center Bolt. Spring shops have an "Ironworker" machine
that can punch the holes. Even shear the leaves down to correct length. Shop we bought our leaves from said their machine was down. Maybe they wanted to punish us for not using them for the whole job... We bought a special bit at a commercial tool supply and barely drilled the two holes (one each side) before the bit was shot. The Length can be adjusted with Torch, Carborundum Blade, maybe even Bi-Metal Hacksaw.
Our 1983 chassis was 18 years old/90000 miles when we got it. Rear sagged slightly and the Leaves were a huge improvement. I think we spent about $175 on parts back then. Leaves, Center Bolts, U-Bolts, Drill Bit. Another huge improvement parts-wise was new hard plastic end link kits for the OEM front sway bar. We never did the center bushings and never added anything at the rear but those leaves. The other "Big Thing" was we found the TOE was Way Out and set it to a Little IN. Another huge difference.
I did a few other things but those improvements were incremental not dramatic like above: New Front Axle Bushings (Axle to Frame, Radius Arm to Frame), Rebuilt Steering Box (mostly because ours was leaking), New Front Shocks from CarQuest (probably re-boxed Monroe Magnum Gas RV shocks).
I decided a previous owner (we were fourth) had changed rear shocks but not fronts. They were NOT all that hard to change. The 1992+ is much more difficult.
I tried to find a listing/pic of a "Repair Leaf" and all I found was complete spring packs. Made me wonder if "Helper Springs" like
might do same as adding a Repair Leaf. The Helpers are designed for DIY installation, but I just don't know if the outcome would be as good as an additional leaf right in the pack. I DO know that there might be clearance/interference issues in a motorhome installation. Lotta stuff under there!