Just traded a 2016 Sunstar 26HE small class A on the F53 chassis - can confidently say that the seats were terrible. Like sitting on a piece of memory foam with another piece of memory foam behind your back. Neither was very comfortable. They were faux leather I believe and did not breathe well either.
Had a 2002 Roadtrek before that, that I believe had the Chevy Express van seats in it, and those high back buckets were great. Very plush and comfortable for all day driving. Cloth seats work better for me.
Am looking forward to the Sprinter seats in our new rig. Everything I've read about them seems to conclude that they were well designed for all day use. I hope so, because we like to cover plenty of ground to get there, wherever there may be.