Are the chassis different? MB vs Chevy, for example? I tend to think of seat comfort as being a result of the van manufacturer. That said, sometimes the upfitter buys the base model van and sometimes they buy the upgraded version and that definitely has an impact on seat comfort.
I also think different seats/driving positions work better for different people. I find Honda seats excruciating, but Toyota seats are comfortable. My spouse would not buy a Promaster-based RV because of the seating position, but finds the Transit fits him just fine. You may just not be the geometry the seat was designed for.
2018 Coachmen Crossfit/Beyond