I've tried several different combinations of helper springs, air bags, new heavier leaf springs, heavy duty air bags, on several style vehicles.
1990 Nissan Hardbody pick up dually king cab with 7-1/2 foot cabover camper
1988 F350 dually 4 door w/12-1/2 foot cabover camper, set up to tow 24' enclosed race trailer
2005 Toyota Sienna mini van set up to tow 16ft open car trailer
1995 E350 EB camper, set up to tow (current)
Below are some things I've learned:
Passenger vehicles are sprung soft for comfort (including that Nissan pick up of mine). You can search out lots of band-aid helpers, but in the end, that's all they are, a band-aid. Those U bolt on Hellwig 'helper springs' that the RV place recommended for my Nissan for instance, when I bought my 6 pack cabover, were a joke! They reverse-bent the factory Nissan leaf springs on our first outing. I added light duty airbags and Rancho shocks, and it still sucked! Awe, the solutions to your problems that you let yourself get talked into when you're 25yrs old and inexperienced
Airbags come in several flavors, from light duty, to the bellows looking 'big rig' style heavy duty ones. If they don't take 100psi or so, they are light duty, and won't help much. The big rig double bellows style are the ones you want for a full size van. That said, I added a set of light duty helper bags to the inside of that Toyota Sienna mini van's rear coil springs, that made all the difference in the world. Used in conjunction with load leveler hitch and springs (between the trailer and van), they aided in restoring most of the ride height, and controlling the 400lbs of tongue weight, and full load of people and race gear. Unloaded and not towing (95% of the time), I just deflated the bags to 5psi, and it rode like a magic carpet all over again
Airbags like the ones UJOR sells are great! Floating the bottom on pads works perfect when offroad, doesn't effect anything while towing down the highway. I robbed the ones off my F350 dually, modified the brakets, cut, weld, trim, and just left the bottom bolts out, they work great. If I didn't have these left over, I would have just bought the UJOR kit and be done with it.
There's nothing as good as getting custom springs made for your rig, they aren't as expensive as they seem. You can cobble your own together from used parts, if you have the patience and skill, a pair of steel toed boots and a penchant for heavy duty 'hammer and prybar' type manual labor helps, too! I first tried to beef up my old spring pack with a second set of Nissan leaf springs, replacing my bent leafs on my brand new truck, mix-n-match style, and added ride rite helper bags, but that was still too light duty. Eventually
I used A1 Spring in Oakland to make a set for my old Nissan, that worked out fantastic with the camper in place. But once I took the camper off, it rode like a buckboard, I had to load up 500lbs of rail track to get a half decent ride out of it, and keep from loosening every filling in my mouth.
My E350 EB camper has an add-a-leaf in the rear, and double bellows airbags. At 50 psi in the bags it handles nicely. When towing my 32foot enclosed trailer, I pump 'em up to 100psi. That along with the load leveler bars mostly restores rear ride height.
I'll stop blathering on, I hope this helps.