Doing dishes is a huge water-user. Best way I've found to reduce water usage is to 1) reduce dishes and 2) only wash once a day. I wash everything, drain the wash water, then rinse everything by spraying it down in the sink, rather than rinsing under running water. Using a minimal amount of soap helps to reduce rinse water needs.
We use a designated drinking glass for each person, no more than 2/day. Use paper plates. For sandwiches & snacks use paper towels, they are cheaper. Cook ahead of time and take food with you as much as possible--soups, stews, etc are convenient, and can be heated up in the same bowl you eat from in the microwave.
One thing you can do is dump the gray tank in a bucket and pour it down the toilet. Quick to do in a pinch.
Depending on where you are, some areas allow dumping gray water on the ground or have places to dump it in special sinks. The latter are mainly set up in campgrounds with lots of tent campers, for them to dispose of dishwater.
There are lots of posts on here about connecting the gray/black tanks with a Y connector and equalizing the levels. That may take a little rigging up, depending on how your tank outlets are arranged, but it generally works OK.
