Ditto Twin Boat and be sure to take pictures before doing anything. I mean lots of pictures. Too many reports of odd behaviors after doing things like this caused by one wire being dropped behind something.
As suggested it sounds like your battery combining system is kicking in and you may be running on your engine/chassis battery versus the house batteries.
Are you connected to shoreline during this test/
Certified Senior Electronic Technician, Telecommunications Engineer, Telecommunications repair Service Center Owner, Original owner HR 2008