First - nobody is laughing. We all asked these questions when we were new. This is the place to ask - ANYTHING!
Almost all electrical appliances can be run on (a) shore power(plugged in), (b) inverter (batteries), or (c) generator power.
Regarding using the inverter/battery as your power source:
Here's a good rule of thumb: Anything that produces heat needs much more power - microwaves, hair dryers, coffee makers, electric space heaters, electic blankets, etc. They will run your batteries down very quickly through the inverter. The water heater and refrigerator will be, or should be, running on propane.
Other things like a light, TV, or radio can run for longer periods.
When dry camping(camping without hook-ups), you will learn what drains your batteries and how fast. If you are in a location where operating a generator is not a problem, then you can run it when you need power. In many dry camping areas, like BLM areas, generators are not allowed between 10:00 p.m. and 6:00 a.m.
Your furnace, is the most important thing to guage. In my rig, during the winter in the Arizona desert, around 10 P.M., I can have a light on, watch TV for about an hour, have the furnace on a moderate setting, and the batteries will make it to morning. At that point, I must run the generator to recharge. Keep in mind that the batteries run the furnace fan. The heat is produced by the propane burners. (If your rig has a heat pump, it is an electrical appliance and will run down the batteries very quickly, if it operates at all on battery power. I do not have a heat pump, so I can not give reliable info on their operation.)
A lot depends on what you are using and how new your coach batteries are.
Test your capacity or battery life at home before you hit the road.
'03 41' Friendship & 24' trailer with HIS '03 H-D Heritage Springer, HER '15 H-D Freewheeler Trike, and '08 Smart Car.
"Adventure is found in the journey, not the destination."