If you're grid heater is going out, you would have lost the relay under the passenger side battery tray already.. Been there, done that. Dropping to -12v has always been normal in my truck, especially when well below 25F.
Oh yeah, huge draw until truck gets up to speed. Note how the lights in the cab will dim until the truck gets up to temp and the grid heater stops cycling.Thanks UtahBruteForce, I just don't recall there being that much of a draw before!