it does sound like a fuel problem but generally if the o-ring gets stuck in the opposite coupler (on the truck side) it doesnt allow gas through period because it cant push the valves to open the connectors and the truck wont even run, (after it runs out of gas in the lines that is).

not ruling that possibility out totally but seems unlikely to me.
checking for fuel delivery to the lockoff is easy enough by cracking a line and turning the truck on or manually pulling the vacum on the filter/lockoff if it has one of those.

if you get lpgas out of the line during that test? then i'd say the problem is somewhere else.