Traveling in a van for a couple years to see the countryside is my retirement dream and it’s getting really close, so excited!
But gone are the days when i can perch on some little wooden slat and not have my back flare up! 
Any advice/recommendations from older folks on comfort and ergonomics for mattresses and seating (and i plan to have a partition so swivel seats is not an option).
Thanks so much.
We went with 9” of high density memory foam for our mattress cut in 3” layers for easier handling, custom cutting and cost, basically three 3” toppers. Our drivers seat was getting a little soft and lumpy from time so I had that reupholstered with a firmer foam. While on the road, an electric heating pad for driver or passenger seat powered by a pure sine wave inverter.
I use comfortable fully mesh camping chairs for many years camping or staying in a rental and refuse to use anything else
Walk. Average several miles a day.
I just discovered glute bridges, and they make an incredible difference. I tried them one day, and my back pain was halved the next.
(I have no medical expertise, but I do have this layman’s opinion: The pictures and YouTubes of people arching upward while doing glute bridges scare me. I want a straight line from knees to shoulders, not an upward arch.)
I put a Lazy Boy loveseat in my last RV. But that RV was bigger. It had a slide. For my van conversion, Imma shop for a smaller-but-comfortable substitute.
You may want to reconsider that partition. If your back likes the cab seats, I don’t see how back health can be less important than having the partition.