I'm not sure this totally relates, but our old gas coach would have problems especially after filling up and hitting the asphalt on hot days. If the fuel came from cool underground tanks into the warm coach tank, then we drove out onto hot pavement for a short trip and then parked, we could get a strong gas smell. The small vent line followed the fill tube up to the fill port, and into the metal neck just below the cap. If you used a non-venting cap that would be a problem, and Fleetwood issued a cap recall at least once that I remember on the thing.
The vent tube was rubber and could fail with age, as could the large fill tube. On one occasion I found a large sag in the vent, and it had liquid gas in it. I repaired the tube by splicing new fuel line in (I couldn't access the tank's top, but the old line was good up to the sag) and rerouting the straps they'd used to hold the tube up. I also added larger hose over things to ameliorate both hoses rubbing on frame members, a preventive fix often used for our Beaver coolant system hoses.
So if you find a dip in your vent line, it could have expanded fuel blocking it if nothing else. One time I found an oily substance on our concrete pad, and upon examination there was the same stuff on frame members behind the genset. Noting a narrow tube coming forward from above, I assumed it was the main tank's fuel vent and where the leakage emanated. Turned out that tube may very well be my fuel vent, but the slow dripping gunk wasn't diesel; it was from a tech not making sure the genset's filter was snugged up after its initial oil change. Travel had thrown genset oil back and up. That's probably not relevant here, except to note where a fuel vent may be on some rigs. I certainly appreciate Gerald's and Fred's experienced input.
I would add that at Pacific Pride (Pioneer Fuel) outlets I go to, there is usually at least one low speed pump because they cater to all commercial entities, not just big trucks; company vans and pickups and small church vehicles do business there. Since there is rarely anyone else or perhaps one other vehicle there, I never feel rushed, don't have to use the high speed pumps if I don't want to, and never hold anyone else up. It is totally the opposite of the chaos I inevitably encounter at the dreaded Flying J.
-Joel