My dad has an 05 dually 4x4 4 door with approx. 25K miles on it. Has 325/505 engine (build date Aug. 2004, which makes it past the tsb for mileage, etc. , according to the dealer), 48RE transmission, 3. 73 gears, truck is bone stock. The problem is mileage from a low of 4, pulling a 22' CC boat to...