Well, it's not really called a back-flow preventer....it's the gate valve.
I guess it would be possible for a severe rainfall to cause the campground sewer to force water (sewage) back up your sewer hose and enter the tank. But it would take enough pressure that I would think your sewer hose would have been forced out of the pipe first.
At any rate it is normal to leave the black water valve closed until full or you are ready to leave. And it is also a good practice to leave the grey water valve closed as well and only drain to 50% when filled. Good to have that grey water to rinse the hose after dumping the back water.
Hope this helped.
Chuck - Sparta, TN
2012 Jay Flight 22FB, 2 x Honda EU2000i
2013 GMC Yukon XL Denali AWD
EDUCATION is what you get when you read the fine print.....
EXPERIENCE is what you get when you don't.