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The bathroom vent is the wrong place for a CO or LPG alarm. CO/Smoke detectors belong in the sleeping areas, near the ceiling. LPG detectors belong near the floor because propane is heavier than air.
So it probably wasn't a CO/smoke detector or LPG detector. I'm not familiar with any RV bathroom vent model that has an alarm function. Any idea what make and model it is?
On edit— It's possible that it's actually a marine vent, not an RV vent, in which case it may be set up to sound an alarm if water is coming in. Taking on water through an open vent would be a bad thing in a boat.
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