Same thing happened to us. TT 2 years old, raining, so everybody crowded into trailer wet. Propane alarm goes off and would not quit. We had been camping for a couple days already with no problems. Cut wire to detector. When we got home with propane shut off for return trip and windows open to vent the inside, I reattached the detector - same thing, alarming.
Bought a new one, installed it, opened the propane valve, and all was good.
I guess it just happens sometimes.