Fixed!
We had the RV in the shop for warranty service last Fall. When the techs replaced the underbelly panel after fixing a fresh water tank problem, they pinched the fuel line between the frame and the panel. The problem didn't manifest itself until several weeks after the service was performed - the line must've been slowly collapsing. I'm glad I caught the problem and corrected it when I did. The line was damaged and would likely have started leaking soon. I cut about 1.5" out of the line and spliced it with a hose barb.
Here's a picture of the pinched line:
This is the second issue we've had with that fuel line. It's a poor design - roughly 25' of rubber fuel line just floats free in the underbelly of the RV. I'd prefer to see a hard line run most of the way from the tank. Lacking that, the rubber line should at least be held captive by some clamps every few feet. That'd be cheap insurance against kinks, cuts, and abrasion.