Scott,
You have become a maintence freak

You haven't even had the truck a month and you already have changed nearly every fluid on the thing

Don't forget brake fluid, power steering fluid, and the front and rear diffs. Seriously though it sure seems like us Cummins drivers do take great care of our rigs.
As far as the antifreeze goes, I think it is pretty difficult to get all of the 6 (or 5 1/2) gallons of old freeze out. I was able to get 5 out of my truck.
-I jacked the rear of the truck up
-Drained the radiator, about three gallons
-Pulled the lower radiator hose off the water pump (this was about 1 gallon)
-Pulled the heater hose off going into the heater core and blew air into the hose, this pushed some coolant out of the water pump
-Flushed the heater core with water through the heater hoes I removed until clear out of the water pump
-Flush the heater core with coolant (John Deere Cool Guard 50/50 pre-mix) until coolant came out of the water pump
-put it all back together and fill with coolant. I actually used closer to 5 1/2 than 5 gallons to refill, but I figure some was lost in the flushing process, so I estimate about a 5 gallon change
Don't forget to check again after several miles/days of dirving. It take a while for the last bit of air to get worked out of the system.
I hope this helps,
Ryan