#1 DEF is inexpensive. I don't have a clue how many mpg's of DEF I'm getting, its inconsequential.
#2 your DEF gauge is not a gauge at all. It is wildly inaccurate. You will get a warning when you are within 200 miles of being empty. That is the only accurate measurement I have seen. Therefore, it is quite possible (even likely) that when your gauge read FULL, it was actually significantly less than full.
#3 I wouldn't buy DEF from the dealer, Mopar doesn't make their own, but they charge a lot more to slap their sticker on the same bottle.
#4 Try to buy your DEF from busier diesel locations. DEF has a shelf life, which isn't normally an issue... but if its been sitting on a shelf for 12 months because of a low volume of diesel customers, it may become an issue. I buy mine from my local diesel station that goes through DEF containers pretty often.
#5 Don't worry about DEF usage. Your truck runs cleaner and gets better fuel economy for a minimal expense of adding DEF. Fill it up and enjoy the drive!
2015 Ram CTD
2015 Jayco 29QBS