a couple of possibilities
My first thought was mineral buildup in the head discharge line that's reduced the diameter enough to create enough backpressure to cause the symptoms you describe. You're in fresh water, but that doesn't entirely rule out mineral buildup...just makes it hard water, not sea water minerals. Not only can they build up in a hose, they can also build up on a joker valve (see photo...this joker valve was removed from a toilet on Kentucky Lake).
Whether that's your problem or not, if it's been more than a year since you replaced the joker valve--I'd bet you've never replaced it), it needs to be replaced anyway, 'cuz if it's as worn as I suspect it may be, it doesn't have to be mineral encrusted to cause the symptoms you describe. So remove the discharge fitting from the pump and replace the joker valve. Search the HM forum for "Joker Valve 101," author, me, to learn why, and why joker valves in manual toilets need to be replaced annually, rebuilt every 5-6 years.
The vent line is still a possibility...a filter in the vent line can create a blockage, especially if it gets wet. So if you have one, remove it. The vent thru-hull may be of a type that doesn't allow enough air in OR out...there may be sag in it that's allowed liquids to pool...that'll create blockage a pumpout can pull out, but a toilet doesn't create enough pressure to get past. Or there may be a buildup of waste in the vent at the connection to the tank--both the vent fitting on the tank and that end of the vent line.
All that said, 13 years is at least 10 years past a Jabsco manual toilet's expectancy...I wouldn't rebuild it 'cuz a service kit is about 80% of the price of a whole new pump for it. So I'd replace the pump, but not with another Jabsco...the ones they make today rarely last more than year or two. So this could be the perfect excuse to upgrade to a Raritan PHC. Check this out:
sbo.com PHII PHC LBA
Bottom line: You shouldn't have to replace the vent line unless you just want a larger diameter vent--which is a good idea, btw. Cleaning out both ends of it should be all you need to do it. Replace the joker valve--and maybe the flapper valve in bottom of the pump too...check the head discharge line for mineral buildup...and if it needs more than that, replace the toilet.
