I ran into that exact situation on our last trip out earlier this year. I haven't changed my solar system for years but for the first time my battery light came on when I started the engine. I had the rig plugged in all night so the batteries were topped off and didn't require much charging. I glanced at the volt meter and it was pushing 15 volts! I turned off the engine then thought to look at the voltage coming out of the solar controller and it was 15 volts. I disconnected the current coming from the solar controller and restarted the engine, no light.
I shut down the engine, re-connected the solar and waited a few minutes and the voltage from the controller dropped to 14.6 volts. (What's recommended for my AGMS) I re-started the engine (2005 Ford F-53 chassis with V-10) the light stayed off. My first thought was that the controller will equalize the batteries automatically once a month and that was the cause of the high voltage coming out of the controller so I am going with that, but I could be all wet!