Hey Cisco.. I have the same truck and have had it for a long time.. Only have eaten one trans and that was after 230K miles!! So I like to think I got it pretty figured out.
The only time I downshift the truck manually is when descending a steep grade, with a load, and I don't want to slaughter the brakes.
With that said, when towing, I keep it in drive, tow/haul on (overdrive off). These trucks could use a gear vendor but since they don't have one if you have a heavy load and long uphill it will shift up and down depending on the hill and load. I have NEVER had to stick it in 2nd or 1st gear to go uphill.
Make sure your clutch fan is good. You will hear it kick on when you are holding a gear for a while. When it starts to go on and off a bunch you know its going out. It should kick on and run until you shift gears or until it cools off.
Everything on my truck is STOCK in terms of powertrain. I have no turbo kits, fuel programmers, etc. I have stock intake and exhaust. I have never had a major problem. Only on my second trans like I said earlier. I have about 265K miles on it now. I service regularly.
If and when you need service, I have a fantastic shop ran by friends who are experts with 7.3 PSD's. They are a couple miles from NCY.
Let me know if you have any other questions.