Traded my '17 2500 6.4L hemi in on a '21 2500 CTD. 6.4 got 14-15 mpg empty and 9-10 mpg with my camper. Owners manual said regular 87 acceptable, 89 recommended. I used 89, tried to use 87, sucked mileage and performance. The new truck gets 19-20 mpg empty and 11-12 mpg with the same camper. And...