I would not carry extra DEF either.
For one, the truck can go thousands of miles on a full tank of DEF, and you can purchase DEF at just about any Walmart or convenience store. So the chances of running out in an area where you cannot get more within 50 miles is slim to none unless there is gross negligence on your part. I would worry more about running out of fuel than DEF.
Secondly, if you live in area where the temps get above 95F often(like many southern states) then the shelf life reduces to about 6 months.