i have a 2013 chevy 1 ton dually, doesel of course. after plugging in my block heater over night, it seemed hard to start. up untill now everything seemen fine. i do have 120 volts at my plug in.
I almost never plug in my block heater. My Duramax has started many times in below zero temps without it. I think you have other issues to deal with. B.O.