So are you saying you DROVE it 10 miles and back, and the batteries were dead?
OR
You are saying you left, drove 10 miles, and came back to where it was stored, and the batteries were dead?
There is no way the batteries went dead in that amount of time without starting a fire. They had to have been very nearly dead. I agree that they had to have been dead or nearly so when you started out, AND your alternator is not working.
The alternator may not have fully charged the batteries in 10 miles out-and-back but for sure you wouldn't have lost any ground.
That or the batteries are just fine but have become disconnected from the vehicle due to a bad ground or other bad connection.