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.
There's really no way having it plugged in can make it harder to start. If you're having to crank it more, you may have injector, HP pump or something else causing pressure issues.