Mike : I had the same kind of blockage last year in the hose from head to the "Y" valve - it took 22 years, but it was stopped SOLID. We frequently use "toilet bowl cleaner" and sometime a shot of bleach - it still clogged up, but it was only that one hose!?!
Solution - I replaced all of the hoses and the "Y" valve. You might also consider double clamping all of the hose connection along with each thru hull.
Oh dear... NEVER use household chemical bowl cleaners, bleach or any cleaners that contain bleach in a marine toilet...they're highly destructive to the rubber parts in toilet pumps and weaken hose resistance to odor permeation. Other no-no's include pine oil cleaners, Lysol, Vaseline or any other petroleum based products.
ABYC standards call for double clamps on all sanitation system hose connections and/or any other lines connected to below waterline thru-hulls. Screws should be 180 degrees apart...or at least 90 degrees if access won't allow 180.
Re macerator pumps and direct overboard discharge of tanks and toilets: It's illegal to flush untreated toilet waste directly overboard or dump a tank in ALL US coastal and inland waters...you must be in open ocean at least 3 miles from the nearest point on the whole US coastline or nearest island to do either. However, the discharge of treated sewage from the USCG certified Type I or II MSD (treatment device, e.g. the Raritan Lectra/San or Electro Scan) is legal in most coastal waters and in most parts of navigable interstate waterways (rivers).