The electrical system in motorhome, fifth wheel and travel trailer recreational vehicles is really two electrical systems: 120 volt alternating current and 12 volt direct current.

The 120 volt system is generally used to run the microwave oven, TV and RV air conditioner. Some lighting may also be 120 volts. To use the 120 volt appliances in your RV you'll need an external power source (campground or home outlet) a RV generator or inverter.

The power for the 12v system is supplied by an onboard RV battery or batteries. The RV's 12v system provides power for most of the lights, the water pump, furnace, radios, vent fans, power jacks and stabilizers, and sometimes the refrigerator. The 12v system is a big part of what makes your RV 'self-contained'.

Your RV will most likely have a combination converter/ battery charger.

When hooked-up to outside 120 volt power a converter changes or converts 120 volts AC to 12 volts DC to operate the 12 volt system. It also charges the 12v RV battery or batteries which operate the 12 volt system when not on external power.

Generally you should not operate the converter without batteries connected because the RV batteries act as a filter of sorts and provides a more stable voltage to the electronic circuits of modern RV appliances.