RV manufactures buy their seats from RV seat suppliers. I am familiar with only one such company called
Pauls Seating in Elkhart, Indiana, only because my motor home manufacture Phoenix USA buys their seats from Paul's Seating. Back in 2007, our two front seats did not come with adjustable lumbar support. But around 2012, we bought a 3rd matching captain seat
SEEN HERE to replace a generic low-back seat inside the motor home, and
IT CAME WITH a knob to adjust lower-back lumbar. If ordering new seats, MAKE SURE adjustable lower-back-lumbar support is included. Also mention that you have serious back problems. The seat supplier just might offer special internal seat-adjustment mechanisms to address your aching backs.
Here is another consideration......
If you do not already have them, universal 6-way adjustable power seat bases could help your back. They will bolt on your E350 floor and your existing RV-supplier seats will bolt directly onto them. Such seat bases are an RV industry standard. Adjusting height and seat-base-tilt might provide adequate variation for improved back comfort. I might suggest you get them first. If they are not enough, then consider replacing your actual seats with ones that have adjustable lumbar support.
My wife and I surely understand your dilemma. Our first motor home was a new 1983 Toyota-based rig
HERE which had the worst front seats supplied by the RV manufacture. At age 25, we were dying after a few hours driving. We quickly resorted to utilizing our sleeping pillows and blankets. After a few years of that monkey business, I took the seats out and added stuffing behind the cloth seat skins for lower back support. The modification helped a lot, but it was never right. We lived with those nice looking but terribly uncomfortable seats for 24 years, till the day we sold the rig in 2007. Oh, and I should also mention that the tufted buttons in the Toyota front seats were like large pebbles in our back pockets. I cut the painful buttons off which is why you see some missing. Those seats were absolutely ridiculous.