Originally Posted by Mr. Dan
I don’t think a black tank flush will make the black tank drain faster. Maybe your valve isn’t opening completely or you have construction trash, etc., stuck in the outlet.
If the above does not reveal the problem, then look at the roof vent. There needs to be good airflow in for the tank to drain properly as well.
One other thought, if you have one of those inexpensive flexible inspection cameras, you may want to snake it either up from the drain valve end or down from the plumbing end to see about a physical obstruction. It's not impossible that the plastic piece from where the cut the openings may have fallen inside the tank and been left. It could be partially obstructing the drain inside the tank.
If you don't have one of those, look for an "endoscope" for your type of cell phone on ebay or Amazon. Should be $25 or less.
I've got one for my Android phone and also a dedicated inspection camera (it was an inexpensive model - around $50). The 2 of these have saved my butt many a time in both the boats and trailer. Makes it possible to see behind obstructions and inside small holes to follow a wire, hose, etc... Beats having to pull a panel off to find it's not really where you need to be doing the work. Well worth having one in your toolbox.