I drove Ford 15 passenger vans all over N.Y.C. for many years, doing film production and they all had the v-8 engine. Hauling 15 people in them was o.k. but quite sluggish going up any grade of hill.
I also drove the Ford cargo vans, and they were v-8s but loaded with production gear and they were slower, compared to an empty van.
If you are choosing between a v-8 and a v-10, go with the v-10 if you do a conversion and make the van a camper. For the amount of equipment, water and other rv stuff, the weight adds up quickly, and that does not include the riders, sports gear, food and luggage!
Better yet, buy a diesel powered van and enjoy much better fuel efficiency. The Ford 7.3 Turbo diesel is a champ!
I like the Ford vans, but the new front grill is a step backwards in the astetics department.
Any way, good luck finding the right van for your application.