I purchase DEF in bulk at the truck diesel pumps, basically DEF is DEF however you purchase it, and is significantly cheaper there than in the 2 gallon containers. My MH has a DEF guage on the dash and the tank only has to be refilled every 6000-9000 miles or so. DEF has a shelf life of about 24 months depending on heat and humidity.
The truck stop DEF pumps are located on the left side of the lanes while MH def tanks on 2010-2013 units are located on the right. 2014 MH DEF tanks are being located on the left now.