Cutting Holding tank access hole

do you not cut an access hole in the bottom tank it will never seal. Are you sure the macerator pump is not bad??? what style macerator pump do you have? If it's a sealand, they have a ductbill that goes bad in them sometimes and they won't move fluid. It would take something very serious to clog a 3" fitting. Try to pump it out with a pump out, that might suck it the other way. Or, Take the hose going to the macerator pump if that's what's clogged, make a fitting or hose with a compressed air fitting and blow it with compressed air (careful not to do too much at once) and see if you can unclog the fitting that way.

If by plastic you mean poly tanks not fiberglass then I would not cut anywhere but a time the top which won't work since you have pressure from th upper tank. The only way to really work with that material is the origianl plastic weld they use installing the fittings. Nothing really stick to that material. A plastic access plate will never seal reliably, maybe a SS on like pictured above... risky

I agree with J, getting a clog in the outlet fitting is very rare even though reading your post i think you mean the clog is in the smaller outlet, not the 3" link.

I'd look at the pump first. As J said, duckbills could have reversed if pumped against a closed sea cock.

If not the pump, you could shut off the valve on the outlet, disconnect th pump, attach a piece of hose to the valve (long enough to extend above the top of the tanks, reopen the valve and back flush with a dock hose.

Are you sure the dock pump out is working with engines auction?

If it does, and if it works for a few seconds and then stops it s almost always a clogged vent. Do you have a charcoal filter on the vent? How old is it? If the tank has been filled, the filter could have gotten wet and the charcoal solidify

No vent, no pump out

You also risk pressurizing the tank while flushing which will cause the tank to bulge out

Most boats have macerator so you can discharge where legal. A must have especially in area with no or very few pump out like the Bahamas

They can be connected to the tank thru a second outlet or thru the same outlet with a Y valve

Try a different pump out station

Actually I have a similar issue here... the pump out at our slip is very slow. Takes 4 hours to empty a 200 USG tank. No blockage, no vent filter (lazzara used two 1 3/4 or 2" vents. Yet super slow. It starts fast and then slows after a few seconds

Eventually it empties. It s not the tank pick up as it is share with the overboard pump thru a Y valve and it works fine. Pretty sure it s the marina system

Looks like the same type as the jabsco POS. The sealand bellow pump is the only one two use

What is the failure? shaft seal leaking back ti motor? Bearing? Impeller? covered in rust?
There near all made the same. Sometimes it the location that kills the pump and not the material to be pumped (wet bilge).
I'm a Orbendorfer fan but their waist pump impellers suck (small Jabsco with plastic insert).

The Jasbsco can’t run dry even for a short time. Considering how imprecise waste tank gauges are you can never be sure it won’t run dry

That s not a problem for the sealand bellow pump

I agree that all of the jabsco and competitor designs like that suck. Sealand T series is the way to go, BUT different configuration AND has a 1/5" discharge hose versus 3/4". Just go to West Marine and get what's on sale and get the extended warranty.

Had a similar problem on a 400 gallon waste tank. We almost never used a pump-out station, just pumped at sea. The bottom of the tank filled with sediment, over time, and the deck pump-out was clogged. The macerator pump-outs were higher than the deck pick-up.The owner was using the boat solo at the time it was discovered, so he found my electrical fish tape and shoved it down the deck fitting. That broke up the mud, and the pump-out worked, again.