I had a similar problem w electrical
and it turned out to be the connections on the main power switch in the cabin. It was a combination of the nut that holds the washers and battery cable eyes on the posts and the resulting arcing after they got loose that gradually dropped the performance on the whole electrical system. The voltage on the panel kept getting lower and lower on the voltage gauge. As soon as I cleaned up the connections it was back to 14 volts and strong performance.
Whatever it is, I hope you don't have to pull too much hair out finding it.
Good luck.
Brian
Hey! I've got an avatar now!