I did it, DEF in the fuel tank

[Rondo]

Even though it can be a pain in the a_s, the DEF fill is under the hood on the GMC and Chevy's. Can't put it in the fuel tank from there. Wondered why they had put it there but was told the heat coming off the motor keep it warmer during the cold weather and it's a short trip from the filler spout to the tank which is just under the passenger seat area outside of the truck.