I just installed an RDS 91 gallon Aux tank/toolbox in my 3500 and, so far, so good. Install went well and the height of the tank allows me to continue using my Truxedo bed cover when not towing. I'm looking forward to never, ever having to fill up my truck with my TH in tow.
By my figures, with the average mileage I get (9.5 mpg), my combined 127 gallons should give me a max range of around 1206 miles until empty. The down side is the 91 gallons will add over 600 pounds to the GVW of my truck. However, my truck's Payload is 5,000 pounds so I'm not worried about the extra weight.
Checking the fill rate to the main tank from the aux tank, I found that approximately 26 gallons flowed in just under 2 hours. At my burn rate of around 14 gallons in 2 hours, there should be plenty of gravity flow fuel to replenish what is used.
