I pull a 12,000 pound 5th wheel camping trailer with my 04 Duramax/Allison. Have to watch my speed as I hardly know it's back there. During visit to a local RV dealership, I was told that the "powercurve" for my truck indicated that the optimum RPM for towing was 1800 RPM. I have never heard of a "powercurve" to serve as a guide for the optimum RPM for towing a trailer. Is there an RPM at which I should drive that yields the most efficient return in terms of fuel consumption and interstate speeds? On non mountainous interstates, at 2000 rpm in tow mode and overdrive, I am pulling my 12,000 pound rig at approximately 65 mph. Any guidance will be appreciated.