I was a patient man with my gas detectors but not anymore.

One thought popped into my mind when you mentioned your gas detector was under the sink...maybe it could be the vent on the sink drain is allowing methane/odors from the waste tank to come through? Also check to be sure any cleaning product or trash bin item could be causing it. Other than that...could be a bad unit or moved to a different location. I'd try moving it to see if it continues to misbehave.