I recently had a conversation on this topic with a transmission mechanic who was rebuilding my friend's van transmission. His unequivocal advice was to get the transmission out of overdrive and into a lower gear whenever there is any significant load. He advised not using overdrive when towing anything and to drop down a gear when going up even moderate hills, particularly when pushing a headwind. This advice was premised on the vehicles in question being class B RVs which are heavy to begin with and also have significant frontal area air resistance. Slowing down is also probably in order.