Our 2007 2499 drains into the Grey Tank. DOnt know if yours is different because it's a 04, but the easiest way to check it to run water and have your grey tank open.
I agree, you might want a little more water in your black tank, but as a full timer, I'd much rather have the blcak tank only hold black, as I dont want to have to empty it more than needed.
We had one of the Black Tank Rinsers installed at Sunline when they built ours, and I'm glad we did. For those that dont have one, make sure you buy one of the hand held wands that goes in from the toilet. You'd be amazed at the amount that comes out after I turn this on.
We did a test to see if we just emptied the tank by itself as to how much more would come out after we turned it on, and let me say "WOW