If it's a new camper, I would either asked the dealer or let them fix it if there is a problem.
Otherwise, -- myself, I would probably climb up on the roof and remove the cover from the stack and run water down. If it's open I would think it would run thru to the tank. If plugged it will run over onto the roof when the pipe gets full.
Maybe one of our plumber members will chime in and correct me, or, OK my explanation.
2008 Cruiser CF26RK
2013 Ford Eco-Boost
Heavy duty payload - Max tow