Oh, never even considered the dump tanks for that. I couldnt think of a practical reason for one, until you mentioned it. :)
Well I already have the tank treatment, though could always use it in my house septic. I currently just buy a big bag of yeast and flush it down the toilet. Cheap and effective.
As far as Dawn, another good practical use for it. Bet they never thought to advertise, Hey you can dump it in your Poo tank so it cleans out nicely! :) I use Dawn to remove the wax from my vehicles paint when doing a clay bar cleaning, before applying Zaino polish. Works great for that too.