I have a 1000 watt whole house inverter wired to 2 6v GC batteries. It is wired less than 2 ft from my batteries with very thick wires but will set of the alarm when the MW or other items brings the batteries down to 11 v. For what you are trying to do I would either rewire or install a new 12 v plug to charge your laptop or phones. My OEM 12v plugs would not recharge my laptop because the wires were too thin. I added another 12v plug with #12 wire and all was well again. I put one plug up front to charge while driving and one in the rear to use the laptop in bed. A small plug in inverter 80 - 100 watts will work well for your lap top and USB .5-1 amp 12 v plug will work well for your phones. I rarely use the larger inverter anymore.
