Hello, I'm searching for information on my 2008 chevrolet dulley truck. I bought a 2014 Road warrior fifth wheel that weighs approx. 18,000 lbs loaded. When stock the truck only got 6.5 mpg. I had a shop do a 4" full exhaust, a custom program deleting the dpf filter and a drop in air filter. Took it to Mrytle Beach for the first time since the mods and only got 7.6 mpg at 63mph. Any suggestion?