When you have both grey/black drain valves open, Is the flow out of black/grey common drain header steady or does it kinda gurgle ??
If gurgles then black tank vent is not allowing air into black tank
Black tank drains using grey tank vent thru drain header....gulping air in
Turn water supply OFF
Try to drain JUST black tank ---open only its drain valve then have someone flush toilet and hold toilet open.......providing a VENT source
Does tank then drain?
Check vent line for blockage.......run garden hose DOWN vent line as far as possible then turn water on with black tank drain valve OPEN
Tanks have to vent...air out as liquids go in----air in as liquids go out
