[travelin texans]

If the previous owner left the black valve open when connected to sewer it probably has the dreaded "poop pyramid", big pile of dried poop, which will be a PITA to remove. You'll need a dedicated hose, not your drinking water hose, to run down the toilet with a pressure tip on it, open the drain valve, have someone inside working the hose up/down/around with someone outside watching the clear section. This may take some time, is not a pleasant job but eventually will work the pyramid out.
Afterwards NEVER leave the black valve open!!
My procedure was to leave the bath grey valve open as the shower fills it quickly, kitchen tank closed, a couple of days before draining the black close the greys, drain & flush the black tank, drain the kitchen tank it can smell as bad as the black, then open the bath tank to flush out the hose, add a couple gallons of water to the black tank + a big squirt of dish or laundry soap & your good til next time. The soap works as well as most chemicals, cheaper & lubes the valves.