Portable toilet / plumbing issue

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This is probably a silly question that I think I have answered myself but just wanted to check with those that are more knowlegable than I.

 

I have a Portapottie installed in my yacht. It is a Dometic / Sealand 974MSD which is a US unit that allows the portapottie to be pumped out via an overboard discharge. I have it plumbed to a macarator then to a below waterline thru hull.

 

The waste tank (bottom part of the portapottie) has a diptube that goes into the tank. The diptube sits a few mm from the bottom of the tank and is cut at a 45 degree angle (will spare the details as to why - hopeully self explainatory!). Top of the 45 degree cut is about 20mm from the bottom of the tank.

 

When I go to pump out the tank will only empty to the top point ofthe 45 degree cut, which means I am left with 20mm of waste that I can't get out of the tank. Obviously when the tank is full the pipe is fully submerged and can therefore get suction, but once it is pumped to the 20mm point it starts to suck air.

 

Question - is the fact that the diptube isn't fully submerged (ie the 45 degree cut out starts to allow air in) the reason it wont suck the tank dry. What can I do to remedy this? Only thing I can think of is to fill it with fresh or sea water every now and again to dilute the bottom 20mm then pump out when on a lean as the dip tube is in the corner of the tank.