It sure looks like the same problem I had
I would still go back to my earlier post and the suspect is the pickup tube. Here's what I did:I pumped using the deck fitting, out came about 5 gallons and quits.I pumped using the macerator, out came about the same amount.I disconnected the breather tube at the holding tank, lifted the end so it is higher than the exit hole on the hull, poured a bit of water in from the tank end, and it ran out of the exit hole on the hull. I reconnected the breather tube to the holding tank, poured water into the exit hole on the hull, and the water never filled the tube, So, breather tube is clear all the way into the tank.I opened the deck fitting, and slowly poured water into it, eventually water came out of the breather exit hole.I also thought about using a snake. Looking at the tank's fittings, the input and output to the tank are both on the side near the top edge. The breather connection is on the top surface of the tank. The output goes from the tank to a T fitting. Since I don't know how to explain the parts of a T fitting, so I have drawn a picture below. Based on the drawing, I could use a snake from the deck fitting and go straight through the T fitting and into the tank. I don't know how your tank is fitted, so you will have to look at it.However, the T fitting is at the top of the side panel, once inside, no way is a snake going to make a turn by itself downward towards the bottom of the tank.Also, once the snake goes into the tank from the output fitting, there is another tube inside, and it makes a 90 degree turn down to the bottom of the tank. This is the pickup tube. You won't be able to make the snake take that turn either.I know all of this to this level of detail because I drilled a 6" hole to look inside of the tank, followed by installing a 6" inspection plate. Now I know more than I have ever wanted to know about the poop tank.Anyway, once I drilled the 6" hole and looked inside, I knew there is no way a snake would've worked.By the way, depending on the size of your tank and how it was designed, there may be a baffle. Yeah, I know this because I drilled my 6" hole right on top of it. So much fun. That added another 3-4 hours of work removing the top part of the baffle so I could installed the underside of the inspection plate.So, I don't think you really can put anything in there to break it up other than the usual stuff from WestMarine, or vinegar, but not a snake.My pickup tube got a hole on the side and it was sucking air after pumping out the top 5 gallons or so. I installed a new pickup tube, it is a PVC pipe + fittings, and all is well now, finally.So, my money is still on the pickup tube.If you want more detailed, step by step info, let me know and I will email you all the steps.