recently when our dump outlet on our heads (two of the four) blocked up, we had a real problem but discovered a handy trick on the Lagoon 450 black water tanks that probably works for a few different boats.
The pump out hose and fitting on the deck aligns directly with the dump out pipe from the bottom of the black water tank. So with a little jiggling you can run a hose from the top all the way through the black water tank and out the bottom past the dump valve (thruhull).
This means if you have a severe blockage , as we did, you can use your Deckwash and under pressure blast your way through the blockage, as you would with a household blocked pipe.
We combined this with some Muriatic acid (neutralised with baking soda) to get us moving again.