The campground paper we got this weekend at the park actually had a blurb in it about how to dispose of your graywater; pouring it in a vault toilet was the second of 2 available options, but I am in Ontario so idk what the rules are where you are.
Check at the CG and see if they are ok with it. I personally think it would be fine but what do I know.......
Maybe the people are looking to see WTH you have in the container. Its not everyday you see someone walk into the 'john' with a large bucket