I can see how the sewage came into the gray tank and filled up to the point where it would come out the shower and sinks. I don't think the OP has any liability at all. It's might be recommended to close your tank valves when your not there but it could have happened when you were there while you were sleeping. This has a yuck factor of 9.... I would not only mention the bio hazard but I would also mention the mold issue in the walls.. The campground is the responsible party here in my opinion. But I am no lawyer...