Hi,
We have a ProMaster 136 inch wheelbase that is just under 18 ft long. We converted it from a cargo van to RV.
We like the length, and definitely would not want anything longer. It drives and parks just like a car. As a bonus, we regularly get 20 MPG. We owned 4 RVs prior to this one over the years and this 18 footer is our all time favorite.
There are even people who use the short wheelbase (118inch wheebase) for RV conversion and who love them. Its all a matter of your personal preference.
If you can't find a class B based on the 136 in wheelbase ProMaster, Wayfaer (
https://wayfarervans.com/select-a-van/) will do a simple conversion of your cargo van. I'm sure Sportsmobile would also do such a conversion.
A directory of van conversion outfits -- might be one near you that would convert the 136 ProMaster:
https://explorevanx.com/
Our van is 8 ft 7 inches including a roof fan.
Gary