Emerald Coast Sailor

Is there a way to drain DEF from a Silverado HD? I had 1.25 gallons in a half emptied 2.5 gallon jug and an unopened 2.5 gallon jug that I put in yesterday and the mileage seems to have suffered as a result (based on the truck's information feed). Maybe the DEF turned bad? I was at 22% DEF level when I added it. If I can't drain it, perhaps adding another 2.5 gallons of new DEF would help?