Aren't all the 12v outlets connected to the same bank of batteries? Of coarse voltage is higher when charging but When you say higher for a few seconds, how much higher? Could you be getting a static charge for some reason when you plug something into that outlet?
Thought about this. Maybe a static charge could be building on the run to the bedroom area. In the past I have taken 2 wires, hot and ground, put them in an electric drill and twist them up so you have a twisted pair. Sometimes that will prevent things from being picked up on the line. Just a thought. Don't know if that is your problem or not.
