Common problem in Jabsco toilets
Due to either a design or tooling flaw, the wet/dry valve is defective. It's just a little "gate" that the lever swings to block or unblock the flow of flush water...in a high percentage of toilets made in in the last 5 years it hangs, creating backpressure that many users mistake for a clog downstream. The only cure is a new wet/dry valve assembly...or a new higher quality toilet.