Voltage meter reads 14 volts when truck is idling and driving at 55 mph. When I start turning electronics on it keeps dropping voltage. With the head lights and parking lights on it reads about 13 volts. Then when I turn the heater on it drops to what's looks like is 12 volts. This is what the volt meter in the dash says I have not checked with a volt meter but I would say there won't be any difference. Any ideas?