2001 Chevy S 10acre gas mileage decrease
1. water and gasoline can not be mixed. Period.
2. Yes, as only USA does winter and summer blends. But then again, we are screwed only as much, as we permit it.
3. not enough data to make any reasonable conclusion. No truck make, model, engine, etc. You definitely need to go back to parts store, but battery cleaning tool and some battery sealant, and use both. Maybe, just maybe, you got hit by winter gas, but my Silverado happily does not care, so I doubt. I'd start with complete inspection - pull plugs, check wires, do soap test. We simply have no idea what are you running, and it can be anything from carb to ECU problem. Even loose gas tank cap.