Propane

Quite simply, the gauge is wrong. Out of calibration.

LPG tanks (and tanks for other pressurized liquids) are filled to 75-80% of WATER CAPACITY. For the purposes of measuring propane, that is FULL. This is what a properly functioning and calibrated float gauge will say. But float gauges are frequently wrong.

LPG tannks (as opposed to DOT portable bottles) are usually filled with the bleeder valve open. The bleeder valve spits liquid when the liquid reaches the FULL level, which is 80% of water capacity. Tanks also have a overfill-protection device on the fill port, which might shut off before the bleeder valve spits. With these two protections, overfilling is unlikely.

There is no way a pump could fill liquid beyond water capacity of the tank, as the hydrostatic pressure would either go higher than what the pump could supply, or the pressure would go high enough to blow out the pressure safety valve, which is the third protective device on a LPG tank.

I have had that happen as well.. Normally it only goes to a bit above 3/4 but one time it went all the way to FULL.

The tank is designed to make overfilling next to impossible.

I do know how to overfill it (Very high pressure pump) but Flying-J pumps are not able to do that.

If they did it right, "full" on your gauge means "full to the point you can fill it" not a completely full tank.

For fixed tanks like those on MH's there is a small bleed valve that should be opened during the fill process. It will bleed gas while the fill is in progress, when the tank is full it will bleed liquid. The valve is 3/4 of the way up the tank. If the fill was done incorrectly it could be overfilled.

Your gauge may be the type that shows full when the tank reaches the 80% maximum which would make half full 40%. The gauge inside my coach registers by thirds but still the same concept. A full reading on the interior gauge will be 80% on the tank. I would use interior gauge only as a guide. Check the gauge on the actual propane tank for an accurate reading. Full=80% on the tank gauge.

Tanks are filled to 80% of capacity. The gauge is most likely inaccurate.