I have an 04. 5 3500 4x4 dully, QC, SLT, 325/600, 48RE, with 3. 73 rear end. When I bought it, I was getting 13/14 around town and 17/18 on the highway at 1850RPMs (about 68 MPH) empty. When truck had 700 miles on it I started towing our boat (triple axel trailer and boat weigh 10,000+) got 13...