Many years ago, before overfill prevention device (OPD) I used to get 20# liquid petroleum gas (LPG) tanks filled wherever. I happened upon a place where the attendant started filling my tanks, and then took a screwdriver to the vent. I wondered what was going on, until fog started coming out the vent, he closed the tank valve, closed the vent, closed the fill valve, disconnected thee fill hose, then charged me by the gallon, not pound of LPG. Later, I found that this was more accurate than what I had gotten before. The same attendant refused to fill outdated tanks (I got them filled elsewhere until the others caught up). After OPD came in, and I had swapped out my tanks, he informed me that 3 out of 5 OPDs were non-functional, found out by filling the tanks with the vent (actually the full vent) open. So, the full vent does not defeat the OPD, however they (the OPDs) are not failsafe, and there is nothing saying that if the attendant finds a non-functional OPD, they have to condemn and drain the tank, or even report such a failure to the owner, I would believe that the OPD on your coach is not closing its valve. You could replace the OPD or find a knowledgeable attendant to properly fill your tank. You probably also could have opened your full vent until the fog stopped coming out of the vent, BUT THAT WOULD HAVE BEEN VERY DANGEROUS.