Get yourself a clear adapter that goes on end of drain line and then connect sewer drain hose to it.
With the clear adapter you can SEE what/how your waste tanks are draining.
This one has a hose connection for backflushing up drain line with water
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Open your black tank drain valve.......
Run a snake and garden hose with pressure nozzle down toilet breaking up clog with snake while flushing with hose.
Continue until water is running clear out drain line.
After you dump black...close drain valve (leave closed until next time to dump) and add a couple of gallons of water back into tank (2 full toilet bowls). This keeps bottom of tank wet so new stuff doesn't stick.
When you go potty fill toilet bowl before flushing so that the water carries the stuff down line into tank.
Dump tank when it gets full.....the high liquid level hels carry/flush stuff out of tank.
Then flush/rinse tank......that's where the clear adapter comes in handy.