Congratulations on the new truck.
Not knowing what the previous owner has done, I would at minimum go through the truck and change all fluids and filters.
I have also had a deuce that when changing out the front boots, I discovered all the bearings were greased and maintained except for the inner right side bearing. It was like they repacked the drivers side and then started the passengers side but quit half way and put it back together. Only way to know is to take it apart and look/regrease. Fully understanding that the work involved to do this on a 5 ton is a lot more (or heavier) work then on the deuce.
BTW, when I emptied the oil out of the transfer case I found a bearing cage that had been chewed up on it's way down. Thus even though it worked fine at that moment, with the bearing cage missing those bearings would have eventually came out as well and I could have been stuck on the side of the road with a big tow bill. This is the stuff that you find out when you change the fluids vs. topping them off.
I will defer to the other SMEs as far as seafoam and injector cleaner type treatments. I would suggest some type of diesel treatment to compensate for the low sulfur diesel that you get now vs. what was available when the truck was built.
There is also a maintenance schedule in the TMs, that would be a good place to start.
