And I'm thinkin' you're right, Nick...it IS air. And the reason it's escaping through the toilet is because it can't get out the tank vent. A new joker valve can't fix that.
As I've posted a bunch of times, the two most common locations for a vent blockage are the thru-hull and the other end of the vent line--that end of the hose and the vent fitting on the tank. If you have vent line filter installed, it's also a likely culprit if it's ever gotten wet due to a tank overflow or just heeling--which is why vent filters are a very bad idea on a sailboat. If that is the problem, don't replace it, just get rid of it.
Start with the thru-hull...use a screwdriver blade, ice pick--whatever works. If that doesn't clear it, go to the other end of the vent line. Meanwhile, don't use the toilet again or try to pump out till you have cleared the vent.
Once you have, now would be a very good time to replace the "vent" thru-hull with an open bulkhead thru-hull that'll let you prevent any more vent blockages by sticking hose nozzle up against it and backflushing the vent line every time you wash the boat.
--Peggie