Hi everybody. Back from the driveway. I confirmed the line I showed was indeed the fuel line by running the generator for 10 min then clamping off the line and the gen sputtered and quit within about 5 min. I unclamped the line and after a short bit of starter operation she fired back up. So that was the fuel feed line for sure. (I had also traced it back again and confirmed that was the only line going to the gen, but the clamp/quit confirmed it).
Anyway, as others have suggested, I also replaced the fuel filter just in case it was getting clogged, but there was only 23 hours on the generator when we bought the rig. But for less than 5 bucks at the NAPA store I had a new one just in case; it was exactly the same size and made of metal like the original (NAPA #3012 if you're interested).
So, to the guts of the story. I had guessed that the fuel line connection in the fuel filler neck of the gas tank had a line that dropped down into the tank. Once I had confirmed for sure the line was the feed, I was willing to make the effort to pull that connector assembly out. Took off the hose clamps, and yep, that's exactly what it was. See the photo below of the metal pipe that then has a nylon line connected to it to drop down into the tank.
I measured the distance between the centerline of the metal pipe and the bottom of the nylon line; it was about 12.5" as you can see by the ruler in the photo. The bottom of the tank was about 14" from the CL of the fuel fill neck line, so I decided to extend the nylon line a bit since ours had shown signs of the line being quite a bit "high" of the 1/4 tank mark.
I had some slightly larger ID fuel-proof nylon line from a previous fuel injection project, and used my heat gun to soften it up and push it up on the black line. Once it cooled it was perfect. Nice tight joint that wasn't moving. (PS: NEVER use rubber fuel line for complete immersion, even if it's fuel-injection line. The rubber exterior of the line is NOT rated for continuous contact with fuel, only the inside/liner portion of the hose. So, like this project, any complete immersion in fuel must be with nylon fuel-rated plastic line).
The last thing I did was cut some little wedges out of the bottom of the line. This way there's no way the line can somehow get EXACTLY flush/seated to the bottom of the tank and shut off fuel flow. You can see this in the photo.
I ran the generator for 15 minutes or so after putting everything back together and it works fine. So now hopefully I'm further down in the tank with the pickup tube and it'll work even down to the 1/4 tank level or so. (I presume the 1/4 tank thing is so you don't run the gen during the night and run out all your fuel so you can't drive away. I can't imagine ever doing such a thing the way we use our RV, but I guess I understand the reasoning).