Bought this truck couple of months ago, had the oil changed 1st thing, had the dega bottle topped by dealer, 2500 miles later added 1&1/2 gallon to bring it back to the min mark. Where did the coolant go?

 

It puked out the degass bottle, most likely the person you bought it from knew about this issue. The most common causes are /rutured egr cooler/blown head gasket/ruptured oil cooler- but you will have a degass bottle filled with oil if it was the oil cooler. It could also be just a leak, make sure it is not leaking coolant from on top of the motor right behind and below the oil filter housing.

 

If there is a white residue around the cap on the degas bottle, bobfbigman is right. Check for leaks first, its easy and a flashlight is all you need. Otherwise it could be slowly escaping through your tailpipe.

If the EGR cooler is ruptured, you will have white smoke from the tailpipe. If you pull the EGR valve and shine a flashlight into the intake and it looks like black slime, and maybe some coolant burping from the degas bottle, you probably need to replace the EGR and Oil coolers together. NEVER one at a time.

*IF* it turns out to be head gaskets (HG), think about installing ARP head studs. The factory bolts stretch and come loose causing HG failure.

Best of luck to ya! Hope this helps!