I have a 2007 Crossroads Cruiser that is always connected to park electric supply. Recently, the lights dim at times, dim more than bright. Could this be related to old batteries? I think they are original, (2) 6 volt in series. I disconnected the park source and the batteries, metered them individually, and they both showed 5.6+ volts. Do I need to replace the batteries or is there an inverter issue? Also, where might the inverter be located?