The reason I said I assume the power cord that came with my trailer is a 25-foot cable with 10-gauge wire inside is because my trailer is parked at a storage yard as I'm doing my research, but research shows that 25-foot, 10-gauge is fairly standard.
I have installed a Progressive Industries EMS-HW30C in my trailer, so I do get a display of the voltage coming in, as well as the amperage being used (it also displays the frequency of the AC and an error code if something is wrong with the power, but that isn't at issue in this regard).
I just had some concerns before spending the money for an extension cord. Where I plan on using the extension cord, I don't think I'll need the air conditioner there... I'll be in the shade and it rarely gets above 90 degrees. I might want to use the microwave though. Primarily, I'll just be using DC power for lights and my MaxxAir fan, and AC for battery charger, refrigerator and possibly water heater. The refrigerator can automatically switch between AC and gas while the water heater I have a switch for electricity and a switch for gas.