Tried to fuel up, but the fill pipe just overflows and runs gas on the ground. No apparent kinks in the line between there and the tank. Is there a filter that could be plugged or possibly a roll-over safety check valve that malfunctioned?

 

Yes, just inside the tank itself where the fill tube enters there is a metal flap for what I would guess would be to make it more difficult to siphon the tank among other things.

I had the same issue after my own tank on my truck was siphoned out. When they removed the hose they bent this metal flap and essentially closed the fill tube. If you remove the fill tube at the tank you should be able to use the handle of a screwdriver to pop the flap back into place.

 

Also take a look to make sure the vent hose isn't kinked.

 

are you refering to the flapper that is visible? or is it in the neck of the tube?
the one that is visable i believe is used as a ground, not sure if it is for when filling or just all the time, maybe both.
if it is another one im not sure where it is located or for what reason.

to the origional poster...

get something stiff but bendable like maybe some copper wire, and see if you can put it into the tank to make sure nothing is jammed in the fill tube. if you have a scope even better...
where are you located? i think i have one that is about 1-2 feet long with a light on the end of the eye. im in toronto ontario area.

 

Apparently it is in fact officially called the anti siphon valve.

It is located inside the fuel tank itself and you will need to remove the fill tube from the tank in order to get access to it. Perhaps having to remove the tank itself depending on your make/model/year of truck.