The alternator that comes on your vehicle is the smallest that the vehicle needs to power them items that came on your car or truck. Where I work we started ordering optional higher current output alternators to power all the extra DC stuff we have to install. I only tried the refrigerator on DC once and the 130 amp output alternator on my Honda Ridgeline could hardly keep up, also the wire going to the trailer socket on the truck is only 12 gauge. Engine compartment temperature will also lower the current from the alternator. Here are some average current draws on vehicles
Air Conditioner 20-21
Electric Cooling Fans 6-15
Head Lamp (Low Beam) 8-10
Heater Defroster 6-15
Pumps, Electric Fuel 3-8