Sounds like you have problem that's very common in Jabsco
manual toilets--a failed wet/dry cam. The wet/dry "valve" is actually just a little "gate" (the cam) that the lever swings to block or unblock the flow of incoming flush water
. Due to either a tooling or mfr'g flaw, it hangs up...creating back pressure that feels like pumping against a clog or blocked tank vent, except it only happens in one mode--usually the wet mode.
The cure: replace the wet/dry cam assembly. Jabsco
often sends out a new one for no charge...but then you have to take the pump apart to replace it, which can be a PITA and really messy because you can't flush water
through the pump to clean it out before you take it apart.
So my advice: just replace the pump instead. It's prob'ly overdue for a rebuild
or at least a joker valve --and besides, modern Jabsco toilets have fairly short lifespan--they're purposely made to be "disposable" like everything else today. A service
kit costs almost as much as a pump, which doesn't cost that much anyway...HopCar will give you a good price
...and swapping it out only requires removing and replacing the 4 bolts that hold the pump on the base and two hoses, which beats the socks off taking the pump apart!