Port fuel tank air locked

No idea of the conditions you're facing, but if you can see the "sag", can you implement anything to remove that dip?

If you're seeing a sag and it's preventing you from fueling I'd say the line is compromise, and simply removing the dip could lead to a blowout. You should be able to disconnect at the tank and pull it up and out. It may be caulked at the deck. I'd be hesitant to install my own fuel line though. Have you tried snaking the vent line? If that gets clogged by a spider web, etc. that could be your problem.

I believe that type of rot or failure can be seen and felt with a close inspection, but often simple temperature impacts of engine heat can allow a hose to shift that isn't otherwise supported well enough, making no statement at all as to compromised hose integrity.

So, then the "sag" in the fuel line is a sag in the vent line? That could be moisture. Suggest removing the vent line from the tank and have it shake hands with some compressed air...

On our 340 we had to cut out a section of the back panel of the compartment under the sink to access the port side vents. On the STB side, we had to pull out the whole couch that ran the length of the salon. Probably not much help for your configuration, but may give you some ideas.

A blast of compressed air could well do the snaking of the vent hose. More than likely some spiders set up housekeeping.

That's my thinking. Either the reported sag has captured some condensation buildup or the vent itself is all buggered up. Blast it out with air, blowing out and not into the tank.

You said you were snaking the vent line. You are aware that there is a screen at the hull fitting so "things" can't get up there.