Confederate soldiers then up in modern day texas - eternal youth

The one I'm looking for is maybe 10 yo I think. It starts in Texas where a genuine Confederate soldier shoots a modern day cop with an old gun. Chase across the hills, girl with soldiers, dying to say moss leads to cave and pool and ultimately eternal youth water hence why the small band of soldiers have survived so far. Help!

Not all the details fit, but this reminded me of Texas Gothic by Rosemary Clement-Moore.

Hunt at the Well of Eternity?