Well the macerator should have come with an adapter to fit the big hole, HOWEVER.. Have heart.
I have noticed many campgrounds, to plug the hole when nobody is using it, use a flower pot full of concrete.
So, I bought the same size flower pot (89 cents as I recall) and filled it with concrete, for the most part.
in the middle of the thing I put a 3/4" PVC pipe (one inch woudl be better)
On top of that is a 90 degree elbow and a common garden hose fitting.
Fits in the hole nicely, can use the "Yummy Sewer Dount" or any other seal you wish if needed, IT has enough weight the pump won't blast it out of the hole.
mine has some fancy plumbing to allow a 2nd hose to attach, If I were doing it again I'd simply use a 2nd 1" PVC and adapters.. (With a plug) but alas, Hindsight is 20-20. The fancy plumbing does make a nice handle though.
I can photograph if you wish.