Another question: are you using premium (high-octane) fuel while towing?
From Engine Operating Tips: "When pulling a heavy load or driving a fully loaded vehicle when the humidity is low or the temperature is high, use a premium (high-octane) unleaded fuel to help prevent spark knock."
Living in AZ, as I do, we tow in low humidity AND high temperatures. When I first purchased my Power Wagon I followed this until someone on another forum said it was not necessary and I was wasting money, so I tried it with the mid-grade 89 I always use (as recommended). Well, the difference was quite noticeable. I had always been able to tow our fifth wheel on 260 on the grade to the top of the Mogollon Rim in 2nd gear. With mid-grade I was down into 1st gear with about a mile to go to reach the top. Back to premium for towing and the next trip (and subsequent ones) I was once again able to keep it in 2nd to the top. This last spring the temperatures were a little cooler and with the price of fuel I again tried mid-grade and again I was down into 1st gear with about a mile to go to reach the top.
