Gem car not holding charge

I have a 2007 e4 that will not hold charge. The batteries are 2012 gel. I charge the car and after about 1 mile it goes down to one red bar and the turtle. I charged it and tested each battery with a volt tester. All registered over 12 volts. I then drove it and after about a mile it went down to the one red bar. I tested each battery and all were around 11.7 volts. I then tested them all together and they registered around 72.4 volts. I then tested under the front hood where they attach to the top of what looks like a control box and they register around 24 volts. Does anyone know what might be my problem?

Clean all connections.

All connector are clean and look new. I even unhook each and checked each one. Still have the same problem. I noticed you member name. Do you have a houseboat? I have been looking for one to buy.

Had 7 of them over the years. Live aboard for 7 years.