[mitiempo]

You said it is an older macerator. Maybe it's worn enough not to get good suction. From your description there is not a valve after the macerator. Do you mean it goes directly to the through hull? Where does the hose to the deck plate come from? I've fixed enough macerators to not want one myself. A hand pump is a more reliable idea. By the way the manual Lavac pump is described by the manufacturer as a pump that shreds the waste like a macerator and never clogs or has the issues of a macerator and they're correct. The same type of manual diaphram pump would work for you. Even a large holding tank would only take a few minutes to pump out manually.
I was planning a larger holding tank as mine is very small and have decided to build a gravity tank in the back of the closet opposite my head. New deck plate above the outlet through hull for easy unclogging if it's ever an issue but from what I have read it shouldn't be.

 

[mitiempo]

Are you putting grey water in the tank as well? Make sure all the connections are airtight. You say there may be a problem at the top of the tank. With good hose and hose clamps (2 if there is room) the seal should be airtight. The three connectiona at the T should also be perfectly sealed. Airtight also means no odors. Good luck.