We had the same types of problems on our 77 center bath. I purchased a flush connection at CW that allows me to hook a hose to the sewer fitting and blast water into the drain pipe back towards the tank. It is clear so you can see the effect and what comes out. I would also use the dump hose as a flush tool.
First let me say DO NOT do this if you are using the slip on fittings without a hose clamp at the sewer fitting. You will have brown trout for sure if you try this with the slip on fittings.
Do an inital dump. Then hook up a non potable water hose to the flush fitting. Turn on the water. Once it flows, Lift the sewer hose up behind the fitting to a point that the water is no longer able to flow out. Once the drain line is full of clear water, drop the hose down to move water out but do not allow it to drain. Then lift it up quickly forcing the water back into the drain and holding tank. Do this 3-5 times. This will act as a sloshing to loosen the solids. Then you can allow the liquid and solids to drain. Do it again, (lather, rinse, repeat) and keep it up until when you dump all you get is clear.
The other option is to pull belly pan skin and install a tank flush system that you just hook the hose to. That is alot of work, and you have to dig in pretty deep to get it positioned correctly.
WBCCI #5501 AIR # 49
1978 Argosy 28 foot Motorhome
Wise men talk because they have something to say; fools, because they have to say something. -- Plato