RE: Back flush vent line
First, make sure there's no vent filter before you decide to back flush your holding tank vent line. If there's a filter, you will ruin it and block the vent.
Second, if there is a downward loop in the vent line (there shouldn't be), backflushing may leave enough water in the loop to block the vent line.
And of course, be really sure you are shooting water into the holding tank vent, not the potable water or furl tank vent.