With the exception of the air-conditioner, microwave oven, an electric heating element in the gas water heater if you have one, and things plugged into the 120VAC outlets, everything in the trailer runs on 12 volts DC. This includes the lights, fans, the water pump, the circuit board, fan motor, ignitor in the furnace, the circuit board, ignitor, and gas solenoid in the refrigerator, the circuit board and ignitor in the water heater, the radio, and the LP detector.
The master disconnect switch disconnects everything from the batteries, except the LP detector for safety reasons, to keep them from discharging the batteries. However, the self-discharge of the batteries, combined with the draw of the LP detector, can discharge them over time.
When the trailer AC cord is not plugged into campground power or an external generator, all these 12 volt
things are powered by the batteries, and none of the 120VAC things work (i.e. there's no inverter, something that turns 12VDC to 120VAC).
When the AC cord IS plugged into an outlet or external generator, a converter (something that turns 120VAC into 12VDC) is powering all the 12V
things AND charging the batteries. The AC cord that is powering the converter also powers the air-conditioning, microwave, electric water heater element, and things plugged into the 120VAC outlets. Keep in mind that not all of these things can be run simultaneously since the trailer power is limited to 30 amps. When using one or more generators, you're also limited by the generator output.
When the trailer is plugged into the tow vehicle, the vehicle alternator can charge the batteries, but not very fast, certainly not nearly what a good converter can do.
The refrigerator is always usable. If you leave it on the Auto setting, it will use 120VAC when you're plugged in, and switch to gas when you aren't. Even when on gas, the refrigerator is using 12VDC electricity. It has a circuit board that is always on, an ignitor that fires when the flame should come on, a heating element around the door frame that reduces condensation (turn that switch OFF!), and on modern refrigerators, the gas is controlled by a solenoid that draws about one amp when the flame is on.
The microwave is only usable when on campground or generator power. That's why we have a gas oven as well as the microwave.
If you will be "dry-camping," aka "boondocking," you want to avoid using electricity, especially battery power (vs generator power) to create heat, and use gas instead. This includes making coffee in the morning when you wouldn't want to wake others up with a generator. You also want to use 12VDC appliances, where possible, such as the TV.
You CAN use a small inverter to power some 120VAC things from the 12VDC batteries, however, it's less efficient than using 12VDC appliances, since some power is lost in the inverter. Using a large inverter to try to power high wattage 120VAC appliances, such as a coffeemaker, microwave oven, or hair dryer, from the batteries, really eats up their charge, and can overheat them. Unless you have a bank of 4-8 golf cart batteries, a generator is best for these. As far as I know, there are no 12VDC satellite receivers, so you'll either have to run the generator OR use a small inverter. Sat receivers use so little power that the inverter isn't a problem.
We use two Honda EU2000 generators paralleled to provide enough power for running the AC and microwave simultaneously when boondocking. Our daily routine is to run them for about 4 hours from 5PM-9PM or so, to recharge the batteries, directly power the lights, water pump, etc (through the converter vs from the batteries), and directly power a TV, sat receiver, or DVD or VHS player. At the same time, the generator is powering all the chargers for the notebook PC, digital camera, PDA, cell phone, etc, which are charging those devices internal batteries.
Hope this helps,