There are a few different energy sources on your motorhome, and different appliances/systems need different ones to operate. What follows is typical; you may have a few slight differences from this.
There's a 12V system for the chassis, which is pretty much identical to that of most any truck. You need it to drive around, but that's about it.
There's the gasoline tank, which the main engine and the generator run off of. The generator is usually hooked up so it "runs out" of gas with about a quarter of the tank left, so that you can still drive to get more gas if you run the generator too long.
There's the house 12V electrical system, which powers all the interior lights, the water pump, the controls for many of the appliances, and various other odds and ends. Often the battery for this system is underneath the entrance steps. Besides the battery, there's a converter that charges the battery from 120V power (when that is available) and also powers the 12V systems when connected to 120V power.
There's a 120V system, which powers the usual 120V outlets and the roof air conditioner, microwave, and can also power some other appliances at times (usually the fridge, sometimes the water heater). 120V power comes from the shore power cord, when plugged in at a campground or somewhere, or from the generator. Sometimes there's also an inverter to produce 120V power from the 12V (house) system, but I would guess that you don't have one unless someone has added one.
Finally, there's the propane system, which is used by the fridge (assuming a typical RV absorption fridge is installed), the furnace, the water heater, and the stove/gas oven. Occasionally the generator is also fueled by propane rather than gas or diesel from the main tank, but that's unlikely for you.
The furnace needs 12V power and propane to operate.
The fridge needs 12V power and either propane or 120V power to operate if it's a typical RV fridge. If someone has installed a residential fridge, it needs 120V power (only) to operate. There are also a few three-way RV fridges that can operate on 12V power alone, or propane (plus 12V power for the controls), or 120V power (plus 12V power for the controls), but they are pretty rare and the operation on 12V power alone is less satisfactory than the other options--the cooling ability is lower.
The water heater usually needs 12V power and propane to run. There are a few models that have pilot ignition and only need propane. Sometimes there's also a 120V element, which requires 120V power to operate (and may or may not also need 12V power for the controls, depending on the model). If you have both gas and electric available for the water heater, they may be used individually or simultaneously.
The stove and (gas) oven require propane and nothing else.
The roof air conditioner usually needs 12V power (for the thermostat) and 120V power to operate.
For boondocking or dry camping, usually one runs on propane and battery power most of the time, and then uses the generator periodically for a few hours to recharge the batteries as needed. (Additionally, many also install a solar power system to recharge the batteries rather than using the generator, at least under circumstances conducive to solar energy.) This presupposes that one can survive without air conditioning. If air conditioning is required, running a generator or plugging in are about the only practical things to do.
When driving, it may be enough to use the vehicle dashboard air conditioner and not need to operate the generator and roof air conditioner. It is perfectly acceptable to run the generator when driving, incidentally, though it does need to be shut down when refueling.
I hope this helps some. There are a few books on beginning RVing that might help explain the systems in a general sense more clearly.
