i was curious so googled this article.....as stated, tanks are made tamper proof....but if filled with contaminated fluid can be drained [did you look for a drain plug?] .... and if you ignore all the warnings, worst is you'll go into reduced power mode til fluid is replenished.
so if you were to fill your DEF tank and see how far you can drive until your power is reduced...then you go by that....i have heard a owner of a DEF equipped truck say he fills his DEF tank every 700 miles.
2008 F250 6.4 diesel with 50 gal aux tank towing a 2015 Rockwood 2109s Minilite.
2015 Miles and nights
2016 miles and nights
feb. 380/7......march 540/3