Any accessories plugged in or connected, even if not "in use"?

Could be failed/failing battery(ies). Try disconnecting them overnight and check the voltage in the morning. They should be the same between them, and close to what they were the night before. Certainly not below 12V. Healthy batteries should last a couple/few weeks of off time before showing any issue. If the problem isn't the battery(ies), at least you'll be able to start in the morning while continuing the diagnosis.