May be a difficult one to answer but here goes, My 2012 350 diesel just turned 80,000 km. Out of this I think about 40k was towing a 8k fiver for three years and this year a 15 k toyhauler. Thinking ...
My last TV had 96k miles on it when I traded it and it still had the original pads with decent thickness left. Probably 50% of mileage was towing. That was a diesel w/o exhaust brake or manual mode. With trailer brakes set up properly, truck brakes should never be over-worked.