I suspect the problem may be that you didn't mount the loop high enough. It needs to be at least 6-8" above the bowl AT ANY ANGLE OF HEEL, not just when the boat's at rest...and if water
continues to trickle into the bowl even when the boat
is at rest, it may not even be above the waterline at all. So depending on where the toilet is relative to the waterline and how far your boat
can heel, that would put it 2-4 FEET above the bowl. Which means you can't hide it under the vanity, which is where a lot of owners of larger boats put 'em.
The owners manual for your Jabsco
manual toilet is here Jabsco Twist 'n' Lock Manual Toilets Instructions
It includes a drawing illustrating how far above the bowl the loop needs to be.
If the loop is where it belongs, you may just need to replace the entire air valve...they aren't expensive.
"If you can't explain it to a six year old, you don't completely understand it yourself."