I would rather not have to drill a hole in the topsides for the vented loop to vent overboard. I am also concerned about how difficult it would be to empty the toilet completely with the vented loop in place, since the toilet doesn't seem to pump water through very well as it is.
Sounds like you are due for a rebuild. The kit will include a new joker valve and part for the pump to make that function like it should. You must have a vent for the holding tank. See no reason why you couldn't tee into that if you don't want a new hole in the side.