Thank you Peggy and Keith,

I have the toilet removed so I have a larger hole for access. I have
been trying to get the sludge to break up. I have pumped a lot of
water into and out of the tank but quite a bit of sludge still
remains. I'll keep trying.

A note to builders: Do not position your washdown raw water intake
seacock close to your holding tank discharge seacock.

I am trying to convince myself to cut off the waste pump holding tank
diptube inside the tank to shorten it. I would be able to barely reach
it inside the tank through the toilet hole. I think it is now about 1"
above the bottom of the tank. Should it be maybe 2"? The tank is
plywood/epoxy/glass and the diptube is glued in PVC.

I had been using some enzyme based additives to try and break up the
sludge, including the KO brand. Perhaps I should have been using it
from the very first when the tank started seeing use about a year ago
but it doesn't seem to do much now. The tank has a 1 1/2" vent.

One thing I did do last night was to plumb in a connection to the
waste pump suction line from the washdown system. Using a Y valve I
can send raw water directly through the pump to rinse it out, it seems
to work great.