I find it hard to believe anyone could go 100,000 or more miles with no trouble on factory original transmission fluid like some claim. Maybe in a little car or something but not in a vehicle that tows something heavy. But, I guess they must have gotten by with it.
I certainly haven't read the manual for the OP's truck (I hope he has) but all the manuals I've ever read for truck tell you how often to service the transmission fluid and filter. It's always more often if towing. I think around 25,000 or 30,000 has been pretty standard with my Chevy's.
I don't have them flushed, just drained and a new filter.