Check the vent again
There two places where the tank vent is likely to become blocked...the thru-hull and the connection to the tank--both the tank fitting and that end of the hose.Something is causing backpressure...if it's not the tank vent, look for a kink in the head discharge hose or a clog. If you've recently had landlubber guests aboard, and failed to give them very explicit instructions about what can go down the head and what can't, one of them prob'ly flushed a wetwipe or something else that shouldn't go down a marine toilet.