Gem Car batteries running down while sitting

[somfas]

As the title says, my gem car just started having a problem with the batteries running down after a full charge, without using the car. I have never had a problem with it and was just about to sell it then this happened. So far I have removed and cleaned each battery connection. I have confirmed that the batteries will hold their charge with the main disconnect switch off. I have disconnected the radio and that didn't help. The batteries are 2 years old and this problem just happened out of the blue. Unplugged it from an overnight charge and its was already down to 72%. Full charge (after the batteries settle) they are at about 76.8V. They will hold that overnight with the switch off. However, with the switch on they will drain to 74.4V in 3-4 hours. Any ideas on what to check for? If there is a short, how can I check for it? Thx for your help. I wanna get this back going and get it sold.