Glad to hear you're happy with your truck, and yes I know what you mean about the pulling power and being happy with it :B Our trailer weighs 16k lbs loaded and the Ram handles it very well.
I would advise you to use your exhaust brake as much as possible, even when unloaded. It will help keep it exercised, minimize soot build up and help save your service brakes. I use mine at all times and only have to apply minimal brake applications even in the city. It truly is a great feature of the Cummins.
In regards to the DEF gauge, it will stay on full for what seems like a long time, and then you will look down and it will be on 3/4 tank. As another poster said, it seems to fall quite quickly once it starts. I haven't let mine drop much below 1/2 tank before dumping a jug in so I can't say how it behaves below that level.
Beware, though; a larger trailer usually follows shortly after ;)