The dealer could not find a problem and all seems OK. I spoke with Forest River and there is no check valve. They thought that the vent pipe may need to be shortened slightly as the vent cap may be resting on top of the pipe thus preventing the tank from venting. I think something may have been blocking the vent line and somehow dislodged. Anyway, it turned out to be a fun topic for disflushing uh uh discussion.
Thanks for all the suggestions.
2011 Rockwood 8285 WS
2011 GMC 3500 HD 4X4, 6.0, 4.10, SRW