gray water drain
If you have a pop up, you might have a 15 gallon gray tank. I have not heard of a tank so small. The smallest trailer we sell has 25 gal. gray and 25 gal. black tanks. Kirk is correct, contact your dealer, manufacture or go on the internet to see what size your tank is. Usually at their website, you will find details on tank size.
I do NOT let the gray drain. Use it as a rinse. The black may not be full, but I dump both tanks at the same time after the gray fills. Black first, then rinse with the gray water. When I am going back home, I rinse the sewer line with fresh water and my hose does not smell.