I've have a macerator pump in my current rig, as well as my previous one. I typically close the gray tank valve the day before I plan to drain the black tank. This allows the gray tank to accumulate some water from dishes, showers and laundry. The next morning, I pull the black tank valve and turn the macerator on. Once the black tank is "empty" I turn the macerator off and leave the black tank valve OPEN. Next, I open the gray valve (leaving the pump off), count slowly to 10, close the gray valve and then turn the pump back on. This process effectively "backwashes" the black tank and you'd be surprised how much stuff it brings out of the black tank. I repeat these steps until I'm satisfied the black tank is clean enough. I've been doing this for the 13 years I've been using a macerator and I've never had a problem.
Bob & Dian
2014 Fleetwood Discovery 40E, Freightliner XCM, Cummins ISL 9 380hp uprated to 450hp, 1998 Jeep TJ