I have a 2021 Coachmen Class B RV on a 2020 Ford Transit 350HD with a 3.5 Ecoboost. I have been very impressed with power and milage. Took a trip to Texas and back, never lacked for power. Just set CC and goes up any hill without slowing, 10 sp trans is smooth. As far as mileage running the speed limit on flats I average 14 to 15. Head winds kill mileage 11 to 12 but I am driving a big sail. Downhill it goes way up. Below is from Jackson CA to home in Stockton CA.
This is a good video on the EcoBoost. You have to put your foot in it now and then to keep the internal waste gates from sticking.
After 6 months I would recommend the 3.5 EcoBoost.