An update to this old thread:
As part of our preparations to move aboard, we're tackling this problem again. Since we're dockbound for a while (traveler and other issues) and plugged tank vents weren't anything that kept us from enjoying short trips and daysails, we had put this problem on the back burner. But to liveaboard, we obviously need a properly functioning plumbing system.
I ran my inspection camera down both fill pipes as far as possible and found no obstructions. The camera is too large to fit all the way into the vent hoses, but I did verify the vent fitting inside the aft tank was clear. Both vent outlets topsides were clear as well.
So we're testing all sorts of methods to check for air movement in the vent lines; finger tip near the vent opening to fill for suction, etc and I decide to soap up the vents. As we slowly filled each tank, I rubbed a soapy paper towel over the vents , and ta-da! Bubbles! So air is moving through the lines on both tanks.
We worked to fill the aft tank first, slowly, and did so with moderate success so long as the flow from the shore side water was a moderate trickle - say about as much flow as if you were carefully filling one of those nozzle mounted fertilizer/pesticide sprayer bottles. More than that and it would back up the fill hose.
We had removed the inspection plate to check the tank level prior to filling, then put it back in place, dogged down. As the tank continued to fill, I noticed the vent stopped bubbling. Almost immediately, Lisa said, "hey...I hear air hissing!" Sure enough, air was escaping from around the inspection plate lid and by the time I get below, with water still flowing into the tank, it was blowing bubbles too!
I'm now thinking that what I presumed to be a clogged vent is actually a failed gasket on the tank inspection plate, at least on the aft tank. If the air inside the tank is escaping through the lid o-ring, then the air will stop pushing through the vent. We opened the tank again and indeed, the water level was above the vent opening. We ripped out the old gasket, cleaned the area on the lid plate, and installed a new gasket. (I happened to have some gasket material that fit perfectly on-board.)
We'll see in a couple of days if that was indeed the problem. We still have the forward tank to deal with, which has had an identical issue, and on this tank the previous owners gooped mounds of silicone over the entire inspection plate. It's easily an inch thick.
I suppose that should have been a clue.