Any y-valve that provides a choice between flushing into a tank or directly overboard, will be somewhere--hopefully somewhere accessible--in the toilet discharge line. If that y-valve doesn't exist, there could still be a discharge port...it would be the overboard discharge for the holding tank. Unless the entire tank is a completely above the waterline (which would make it possible for gravity to drain the tank), there'd have to be a y-valve and some type of pump in the tank discharge line.
Do you have any idea yet as to the make/model/age of the toilet? Marine toilets do have limited lifespans and so do hoses. I'll be glad to help you get everything sorted out..replace what needs to be replaced..."renovate" what can be. Meanwhile, welcome aboard sbo.com...I think you're gonna like it here.
--Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein