once again, someone who got screwed by a RV dealer who sold them way too much trailer for the truck.
if you had talked about this in open forum before making the decision to sell, I would have recommended to do gearing before selling. these motors need to spin to make power and stay cool. 4.56 or 5.13 gears would have taken care of the problem. 4.10s are good to 10k/lbs trailer max or 26k/lbs gross combined.
I routinely towed with 17k-19k gross in my F450 and never had to watch the gauges. unless your new truck is spec'd for heavy towing, you're going to have trouble with the new iron too.
a trailer grossing at 20k/lbs is a heavy tow. F450/F550 territory.
You are assuming way to much and misread the post, I have owned many RV's and am well versed in towing. I tow for my business every weekend. I sized the RV and purchased it from an individual. I put GVWR and meant GCWR which is the weight of the vehicle and trailer with all contents, that is 20,000lbs. Subtract the weight of the truck which is 7000lbs you are at 13000lbs loaded 11035lbs empty for the trailer. I calculated the total GCWR, I sized my final drive ratio with Load Range H tires, along with 5000lb airbags to handle the pin weight. The Hitch was welded to the frame, the ball hitch was welled to my DP HD bumper which was welded to the frame to provide all the strength I needed. My bed could easily have withstood 2 tons direct load and barely touch the overload springs. Gears are great but final drive ratio is what makes the final decision and mine was perfect for keeping the engine in its power band at 67mph. I never dyno'd it but given my mods I was probably in the neighborhood of 200 + - 5 RWHP. The problem here was not weight, or power, the truck made enough RWHP to handle the load. It could handle up to a 5000lb pin weight with no issues. The problem is cooling, these engines are great but once you add boost, and turn up the pump you are creating more heat from the additional fuel. Sure I could back the pump off, gear it up, but then I am pulling at 55mph on flat roads and 25mph uphill at 7mpg's.
Our family outgrew our last RV, and the RV out grew the truck. So its time for something different and modern technology will overcome my idi limitations.
I know first hand of more cummins dually's pulling much more weight than I will getting 14mpg at 70 mph all day long and never getting hot.