I'll check with DH and take a look at the fuild. We had it in and would hope that the dealership looked into the fuilds. I don't think we have changed the transmission fuild since we just got the GMC March/May 2012. We have done two oil changes on it since we got it (from what I can remember).
I forgot to mention that sometimes I feel like I 'smell' burning after hauling the RV for a while. I told DH this and his answer was, "It's a diesel, they smell." which I don't agree with. And at the last two (5 hour drive) - when we parked I heard a 'clicking' noise when the truck was in idle. DH doesn't think it's anything but I do.
It was happening before we turned it to tow only and it almost ALWAYS clunks when we down shift. It's more noticable when we have something in tow but happens even when we are driving (on a highway).
Thanks for the suggestions and keep them coming.