Andrew,
I believe that one of the big engineering pluses of an automatic transmission is it's ability to convert engine horsepower to higher axle torque via spinning the engine just a bit faster instead of, or for awhile before, dropping to a lower gear.
This kind of torque conversion of course makes for superb driveability but is not as efficient as using gears to do it, so it produces more heat (from less than 100% efficiency) in the transmission while this clutch slipping torque conversion is going on. Keeping overall longterm heat to a minimum in an automatic transmission contributes to a longer life.
With lower rear differential gears (as in the E450 over the E350), of course less clutch slippage torque conversion will occur when pulling weight (whether it be only the RV or the RV plus toad). Thus V10 E450 transmission life could be somewhat superior to V10 E350 transmission life when comparing the same weight Class C motorhome pulling the same weight toad or trailer.
