Just so you don't think you were ignored here, can't answer/help on the beep.
It is an inverter (12volt to 120volt) or a converter (120volt to 12volt, to charge the battery)?
How did you eliminate the detectors as a source?
Have you checked the actual condition of the battery(s)? Load-test? Might be replacement time...
Let us know if you find an answer.
2016 Tiffin 40 QBH
2015 38RSSA, traded
2005 TK3 #1869, 10 yrs of memories,
2017 F450 KR--one more Ford is it
2009 F450 4x4-died; 2010 F450-retired