Sounds like someone--a guest?--may have flushed something they shouldn't have( wet wipe, condom?) that's clogged the macerator blades. Or...the boat is a 2008...when, if ever, was the last time anyone checked the condition of the discharge impeller? It may be spinning but too worn now to move anything through the toilet.
You said the vent LINE is clean...but is the vent thru-hull? It's one of the two most common places for vent blockage because it's actually designed to keep sea water out of fuel and water tanks, NOT to provide the necessary exchange of fresh air with gasses in the tank that's so necessary to prevent odor from occurring. Boat builders, who are the worst sanitation plumbers on the planet, use 'em on holding tanks too because it's cheaper than using the right kind of thru-hull for a holding tank vent thru-hull.
So I suggest that you clean it out before you do anything else...use a screwdriver blade, ice pick--whatever works. If it has a screen in it, knock it out...screens become clogged with dust, pollen, air pollution and corrosion and serve no useful purpose in holding tank vents.
The second most common location for a vent blockage is the other end of the vent line--that end of the hose and the vent fitting on the tank. Tank contents can spill into the vent when the tank is over-filled or simply due to heeling.
Does the vent line run straight to the thru-hull, or does it has bends or --worse yet--any 90s in it? That'll interfere with the toilet's ability to flush.
If none of the above is the source of your problem, Paul Campagna is Jabsco's toilet guru...his direct line is
978.282.5246
--Peggie
--Peggie