Here's a copy of the reply I just sent to your PM:
You have two problems...some muriatic acid, which you can get from any hardware
store, should dissolve the blockage. It won't damage anything in the system. Follow handling and use instructions very carefully!
However, 1/4" vent is unacceptably small...we need to explore installing new one. The "standard" size for all tanks
vents is 5/8"--twice the size of yours--and even that's not adequate for a waste tank 'cuz waste, unlike water
, needs to be managed, not just stored.
So we need to explore how to install a new larger vent.