Similar issues; different era: My parent's 5th tow-vehicle was a '73 Buick Electra Limited towing the 3rd trailer, a '77 Ideal 21 1/2 footer. Dad and I installed Air Lift Poly-Aire bags into the rear coils to provide a slight lift to restore the inch or two lost due to the weight of the heavy equilizer hitch reciever, hitch, and a few items in the trunk in order to level it before hitching up and using the spring bars only to maintain the same vehicle attitude with the addition of the trailer tongue weight. These vehicles were undersprung unless one had a trailer-towing package, which this one didn't. With the right amount of air, and spring bars adjusted for whether one had full propane and water tank (inside and also near front) at the time vs. a few days later with a full holding tank in the rear, ride and handling were extremely good, also due to the heavy Buick's very low center of gravity. But, after dealing with air bag tubing that would somehow chafe and leak no matter how I mounted and routed it, and air bags that eventually did the same due to our only needing a little more inflation than the minimum required to prevent chafing, we gave up on them. I concluded the best thing would have been rear coils slightly stiffer and taller, just enough to make up for the additional weight of the hitch receiver, etc. identified above. Too much air easily made for an overly tall rear ride height and even though air is progressive in firmness, it exceeded the damping ability of the shocks and would make the rear of the car bounce like a dribbled basketball. Ultimately, a 3/4 ton '82 Suburban was purchased due to the Buick's chronic overheating issues, and the firm suspension of the Suburban, and no air bags, air shocks (an earlier experiment in the '60's), etc. made hookups easy and not in need of constant adjustment, tinkering, and repair.
I'm no expert, but would guess that the addition of a very thin but highly arched leaf would aid the PCs, and not add too much firmness or too much ride height. You're probably only needing to make a fully loaded vehicle have the same effective spring rate as the dry one, and restore ride height to what it was when dry, plus another inch for good measure since many report their E-350-based units sag about an inch or so. On most with an E-450, I couldn't imagine any more firmness (effective spring rate) being desirable except on the heaviest models, and ride height change needs would be mixed. So, talk with a good spring shop or someone else who also deals with suspension changes and go over your options, which might be supplemental air systems, additional leafs, re-arching of your existing leaf springs or some combination of above, or completely new, different leaf springs. On my father's '97 Roadtrek (Dodge Ram Van 2500), Roadtrek originally added one thin leaf to the driver's side where more weight is. According to some experts, some of these had it, some didn't. So, all things are possible.
