Only have 1 final issue on the head with resistant when pumping in the dry mode but am looking through the post regarding that.
It may just need lubrication. If that doesn't solve the problem, it could be the bottom valve or a problem that's been very common in Jabsco Manual toilets for quite a few years: 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, but can be the dry 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 if 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!