If the voltage dropped to 10.8 with no load, you either have a short somewhere or your batteries or at least one of your batteries is/are toast. Resting battery
voltage should be around 12.8 volts. If you have lead acid batteries, a cheap
hydrometer will isolate a bad cell/battery. Best would be a load tester but you probably wouldn't want to spring for the cost of one of those. Pulling the batteries and taking them to a battery
store would give you access to their test equipment
. You'd have to do that anyway as it's not good form to drop the bad battery(ies) over the side.