I left the battery switch on for about a week.... (lesson learned). My batteries showed between 6 and 8 volts. I figured they were toast. My local mechanic (who was a mechanic in the army years ago) told me to try and charge them each individually for at least 24 hours at 6 to 10 amps.
I did as suggested and two of the four showed 12.9 volts and held the charge. The other two would not hold the charge longer than 30 minutes......TOAST.
My truck now has been reconfigured for two batteries, and a second battery disconnect switch (remembered the lesson). I live at a somewhat elevated level in the mountains of Utah. It is normal to see negative twenty (-20 F) for the overnight low during the winter. Of course a cold blooded NHC250 with only two batteries will not start. I took the advice of many wise and experienced members and installed a block heater (readily available from NAPA for around $70.00).
If it were up to me my course of action would be:
1- try charging the batteries again. If at least two hold a charge you are still in business.
2- install a block heater.
3- reconfigure the truck for two batteries.
4- if none of the batteries are salvageable, then purchase two of the biggest automotive batteries you can afford and install block heater.
5- Drive truck every chance you get. Perma-GRIN is inevitable.