batteries dead

By dead you must mean partially discharged vs dead where the engine wouldn't run. 10 miles is rather short so the batteries must have been discharged before you left and unlikely to have significant charge but some upon return if everything is working.

First fully charge the batteries now as otherwise their life is shortened. Next load check the batteries to be sure they are OK. Next use a voltmeter to determine that the alternator is OK and charging the batteries.

Consider disconnecting the batteries when stored to prevent significant discharge for a number of months with good batteries.

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.

Moved to Tech Issues forum from Towing.