Hello Captain Bill

have you tried running water back down the suction pipe into the tank?  See how quickly it flows in. Another option might be to feed a wire or suitably sized piece of plastic down the pipe. I suspect the pipe will be heavily clogged.  I had a similar issue with pipe from toilet to tank. It was messy but after poking an old fibreglass batten down the pipe the toilet drained scarily fast. 

Typically, there's usually a tee at the outbound side of the holding tank.  Waste tends to collect at this point, creeping up the line to deck plate commensurate to the amount of waste in the tank.  Over time, scale develops in this section of hose.  This is where I'd be concentrating my suspicions.  This is the section of hose that is also the most stinky.  Covered in my write-up in the next issue of Mainsheet.

I had a similar problem when I first bought the boat seven years ago.  A dirt dauber had made a nest in the vent tube (hole in stanchion above the toilet) which obstructed the air flow.  I flushed it with a water hose and haven't had the problem since. 
 
Could it be something like that?
 

The hose from the deck fitting should go to a TEE which ties this to the outlet of the holding tank and to the inlet of the macerator pump.  Your clog is almost certainly in the 90 degree elbow attached to the left of the white tee:  https://www.catalina36.org/sites/default/files/forum/OriginalImage_0.jpg

The problem with TEEs are clogs that develop in sharp 90 degree turns.  Using straight barbs on a Y or a Y-valve would be a good modification, placing the "IN" of the valve at the outlet of the holding tank, one "OUT" to the macerator pump and the other "OUT" to deck.  Keep in mind that a Y-valve selected to the deck position might not exempt you from also locking the macerator pump seacock.  The USCG regulations for Type III MSDs (holding tanks) say that "each valve leading to an overboard discharge must be secured".

On a stock MKII, the overboard discharge is via the 1" outlet of the macerator pump to the seacock.  The way that I comply on Whimsea is via two large WHITE tiewraps.  One goes loosely around the base of the seacock and the other loops thorough this tiewrap and around the closed handle, close to where it joins the body of the seacock.  Once both are tightened, the handle is hard to move to the open position and the WHITE tiewraps make visual inspection easy.  Yes, they have to be cut to operate the seacock, but the use of tiewraps is called out in the regulations as an acceptable locking method.

In my opinion, screwing down the lid covers would not pass the regulations.  The spirit of the regulations (for Type III MSDs) is really about locking seacocks.  Here are the regulations:

http://www.gpo.gov/fdsys/pkg/CFR-2001-title33-vol2/pdf/CFR-2001-title33-vol2-sec159-7.pdf

Nick, as another option to secure the discharge, would screwing down the access panel to the valve be acceptable? Isn't the purpose to prevent an accidental discharge?

Randy and Dave, modified my response above to include both of your follow-on posts.....

Those regs seem to indicate that my Type III MSD (aka my septic tank) needs a certification label on it, but I don't remember seeing one anywhere.  Is that really true?  I'm a 1984 MK1.

Nick, thanks for the applicable regulation. The way our boat is set up, it is not possible to remove the handle or lock it. Also, the use of zip ties is problematic. I may try a hose clamp at the end of the handle and a zip tie to the base. Reading the regs, it seems to me that the intent is to eliminate a discharge by making it difficult to easily operate the through hull. Screwing down the lid accomplishes that and meets the purpose, if not the letter, of the regulation. I am leaning in the direction.

Hey Ben.  Fair to note that Type III MSDs cover more than holding tanks,  Unlike other Type III MSDs, holding tanks represent a special exception to the placard requirements providing they are (A) manufactured with the intent of holding waste and flush water, and (B) are being used exclusively for this purpose.  If you were dumping your used diesel oil into the tank for example, then you would be in technical violation.

Dave.  You might get a boarding officer that agrees with you.  But, you might get a guy that says, "In the regulation, immobilizing the valve handle is mentioned three times; preventing access to the valve, zero.  Skipper, the intent of immobilizing the valve handle is to prevent objects in the vicinity of the valve from contacting the handle in a seaway and causing unintentional valve rotation".

I'm playing devils advocate here.  Fair to note that in my 40+ years of boating, I've never been boarded.....Well, at least in the USA.
 
If you can slip a block wood under the closed handle, perhaps then just screw an eye into the block of wood, smear some adhesive on the bottom, and then set it in place.  After curing, use a tiewrap to secure the handle to the eye.

Thanks, Nick.  Can you point me to where it says that in the regs?  I searched the pdf above but didn't find that paragraph.

http://www.uscg.mil/hq/cg5/cg5213/msd.asp.   Last section "A Type III device that stores sewage and flushwater at ambient air pressure and temperature is not subject to formal U.S. Coast Guard certification if it meets the requirements in 33 CFR 159.53(c).  Such devices will have no U.S. Coast Guard Certificate of Approval, no U.S. Coast Guard letter, and no device label."

33 CFR 159.53(c) simply says the device, "Be designed to prevent the overboard discharge of treated or untreated sewage or any waste derived from sewage."

Thanks Nick. That is an excellent solution for our boat. That shouldn't take more than 30 minutes to come into "technical" compliance.