Stove knobs got bumped to the on position.
OR
The gray tank will also give off an odor that smells exactly like propane.
My experience with this WAS the stove knob got bumped on by putting some storage tubs stacked up against it, detector did not go off because the propane was finding the sink as a low spot. So it never made it to the back of the coach to the detector.
After you check the stove knobs and they are tightly turned off then clean the gray tank. Throw a quart of lemon cleaner down the drain fill with water and dump and see it the smell goes away.