I bought Promaster 3500 RV 2 weeks ago in NOLA and drove home to Florida. First day I got 24.5 mpg, 70 mph, I-10, level interstate, 70 degrees rain and wind. The second day I got 13.5 mpg, 65 mph, I-10, rolling hills, FL panhandle, 60 degrees, windy. Now at 1900 mi ODO I get 12.5 mpg, 70 mph, level interstate, 70 degrees, no wind. I don't know why the fuel economy would drop so much in so short time. The first day it would stay in 6th gear with cruise set 70 - 75 mph. Now it downshifts to 5th gear 3000 rpm and instant fuel economy says 9 - 10 mpg until it speeds back up to 70 mph. It won't even stay in 6th with cruise set at 60 mph.