We have a 2020 Keystone Cougar 27SGS. I winterized the 5th wheel the end of September and everything was fine. I drained the anti-freeze yesterday and ran water in the bathroom, outdoor shower and kitchen sink. The kitchen sink would not drain. I removed the fittings and pushed a 15’ snake into the pipe and had no blockage, the pipe is about 6’ feet at the most from the gray tank. I checked the vents on the roof, no blocked pipes. I put a vac into the pipes and sucked out water but when I blew air from the vacuum into the pipe it built up pressure. The grey tank emptied fine. What do I do next. Thanks.