Quick clean/product for Fresh Water tank?

We never use our fresh water tank except for the occasional bathroom stop on the road. However, we're camping in a place with no hookups this weekend for the first time in years.

There's no noticeable smell or anything and we won't drink from it - we will only use it to wash dishes and whatnot. Is there any product I can add to the tank to help make it cleaner? Notice that although we don't drink it, I don't want to make it impossible to drink as well.

I also won't have time to do the whole fill-it-up and wash-it-out with bleach.