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What about pressure? We had a couple of tanks (20#) in a camper that wouldn't light off anymore in our camper, not even the stove. Still felt pretty full. Took them to tractor supply, they topped them off, 3.3 gallons combined were added. We were like WTF? That's all?
(20# tanks have about 4.7 gallon capacity, IIRC).
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Bad regulator on the camper? Did you try them on something else. Generally, because LP is a gas compressed to a liquid it maintains a fairly contant pressure until all the liquid converts to gas on it's way out the tank, that pressure is significantly higher than the pressure you use it. I don't see how it could ever happen that your tanks became full of LP gas that wouldn't turn to vapor. Either they were contaminated with something with a much lower vapor pressure, the valves are faulty (unlikley for that to happen on multiple tanks around the same time), the refill place didn't refill properly, or the regulator on the "appliance" failed... I'm inclined to think regulator.
To eliminate the regulator try them on something else.
To eliminate the valves, try a known good tank on the camper.
To eliminate the theory that the refill place didn't actually fill them up, weight them (empty weight should be stamped on them). If the weight says they're full (empty weight plus 20 lbs give-or-take) and you tried them on another appliance, then your tanks were likely contaminated with something.
I know LP is a byproduct of refining oil into other fuel products. Is it possible that a refinery mistake put other lower vapor pressure fuels (gasoline etc) into the propane line and some found it's way into a grill tank? Seems very, very unlikely to me but not impossible.