I have a 2014 Eco. The DEF tank in mine is 8 US Gallons. Seriously, mine takes up around 15,000 km to burn 8 USG! So not unusual to have no movement in only 350 miles. Another thing these DEF systems do (it's not just Dodge systems): DEF is basically 50% water and 50% Urea. This stuff crystalizes on the DEF gauge (which is located inside the DEF tank and is not replacable), and this can sometimes cause the check engine light to come on. It causes false readings, eg you can add DEF to your tank and the gauge will not move. Also, the gauge is one of those sonar tape types, and only goes 3/4 of the way up the tank, so until you burn the first quarter of a tank, your gauge will be stuck on 100% full. How I got rid of the check engine light, was to drain the tank (slide under truck, remove fill hose, suck out all DEF with a cheap syringe type hand pump from Lordco or similar..I actually got mine from O-Rielly's for 12 bucks.....very easy to do), and then add just 2 gallons. Then start driving. on day 1 after filling, my check engine light went out. On day 2, the gauge started reading correctly. It's a minor PIA, but once you get it sussed, it's no problem, and I run mine at not more than 1/2 on the guage and it all works fine. Doing this is much cheaper that having a tank replaced just to get a properly working gauge (Coquitlam Chrysler in 2022 quoted me CAD4,300 to provide & fit a new tank). Cheers & I hope the above makes sense!