The vent line is definitely the culprit...it's clear nylon water hose which permeates very quickly. Builders--who are the worst sanitation plumbers on the planet-- use it because it's cheap. Do whatever it takes to get it out of boat. And if it's long enough to require "hangers," it's way too long. So you definitely want to run new one using sanitation hose this time to a new vent thru-hull that's a lot closer to the tank. Dave's suggestion to upgrade it to a 1" hose is a good one, but that'll require installing a new 1" vent fitting in the top of the tank, which is actually not hard to do, thanks to a li'l gizmo called the Uniseal UNISEAL
. I'll be glad to help you spec out the details.