Black tank won't drain unless grey valve is open

Could this be a clogged Black Tank Vent pipe that doesn't allow air into the Black Tank when dumping? Having the Gray Tank valve open would allow air to flow into the Gray Tank from its vent (assuming there is a separate one) or air gets sucked in via the P-traps. I wonder if would be useful to put a tissue over the Gray Tank drains eg. kitchen sink, bathroom sink and shower drain to see it it gets sucked in when the tanks are draining?
I admit these are WAG's on my part.