Originally Posted by woodytwo
May have stumbled upon the solution.
Did a google search and found several RV forums with similar posts.
Problem may be with the vent tube. Apparently all/most chassis have one. comes out of the top of the tank runs along the filler tube to a "T" near the top of the filler tube. overfilling can cause gas to be in the vent tube that gets trapped in a type of "P" trap by a low spot in the vent hose. Gas cannot drain back into the tank and air cannot go up the vent so air goes back up the filler tube causing auto shutoff to click off.
Solution seems to be to realign the vent tube to ensure there are no low spots and drain away gas in vent tube. Several owners did this and the problem went away.
Seems reasonable to me in that I believe that the gas tank is right up between the main chassis rails and filler and vent line would have to be horizontal at some point to fit under the floor and get to the top of the gas tank.
I'll have a look and follow up on findings later.
It is common to have the vent line slip, sag or otherwise develop a problem so if gas gets in it, it forms a trap blocking the air from being able to be vented from the tank. On my 310 I found that the original ties which held the vent to the large fill line had slipped allowing the vent to not ride on top of the fill line, form a pocket and prevent a rapid fill. Once I corrected that, the problem went away, so I would say this well could be the issue on yours. Route the vent line so it is a always going down to the tank so no liquid can accumulate in it.