Almost a quarter amp is a lot, and more than I'd expect as "normal". This is a lot easier to track down than the similar problem in the 24 volt system... It has to be either the Onan or the drive engine... or their 12 volt alternators.
Don't forget accumulated dirt making a part to ground. That would be my bet on this. And if I was putting odds, it would be one of the alternators.
But ground leaks have a way of being infinitely frustrating!
Isla Caja de Muertos, PR