I will stick a factory new jug of DEF in the truck (bed) if I'm going on a long towing trip. The truck sucks down a lot of DEF when it's towing and some of the areas we go through are pretty desolate and you can't exactly pull off and get DEF anywhere. Or fuel for that matter. But other than that scenario, I just fill it up at a pump when it starts getting low. The nice thing about Rams is you get an actual DEF gauge so you have an idea of how much you've got at any point. It isn't very linear but it's still better than waiting for an idiot light to come on.