Work out what you need
"I want a solar system for my motorhome"
"What do you want to run?"
"What is an everything? Can I buy one at the local electrical store?"
In order for solar system to work for you, you will have to have some idea of what you want to run. Without this basic requirement you are wasting everybody's time including your own. A general answer like "everything" will not suffice. It will also annoy your solar dealer!
The first thing to learn is that without spending a fortune on a solar system is that you will probably need to economise a little on what you are already living with, assuming you are currently living in a house connected to the electrical grid. With a solar or other renewable energy system the first things that are ruled out are heating devices like hot water (use solar, gas or wood), the electric stove (use gas or wood), air conditioners (sorry, these things are power hungry but may be possible depending on your budget) room heaters (use gas or wood etc.) and other heating devices or long term high energy appliances like clothes dryers.
Start out with your power bill
Assuming you are currently living with grid supplied electricity you will be getting a power bill every month or so. Look at your bill, determine how much power you are using. Work out where it is going. In Australia you will probably have 2 or 3 different tariffs, one for household power, one for hot water and possibly another for off peak heating or electrical heating. Find out which is which and what is using what! You may need to contact your power supplier for hand here but what you need to finally arrive at is a daily average in kilowatt hours (kWh). A kWh is 1000 watts for an hour or the equivalent of having a 1000 watt radiator turned on for one hour.
Eliminate everything you don't really need
With a solar system, usually the first things to eliminate are heating or cooling devices. This means radiators, electric stoves, electric hot water systems and air conditioning. These are heavy power users and if at all possible you will consider gas or wood to meet these needs, or in the case of air conditioning you may consider insulation, evaporative cooling, shade and open windows.
Consider what you really need
Start looking at all your electrical appliances. Become an electrical sleuth! Every modern appliance and most older ones has some sort of placard on it stating the power required in watts or the current the item draws in amps. Remember current x appliance voltage gives you watts which are the measure of energy used to calculate your electrical load.
Fill out a Load Chart
A load chart is a list of the appliances you wish to run, detailing the times they run and the load. You use a load chart to calculate "your everything" and a correctly filled out load chart will not only calculate your solar array, but also your battery bank and inverter sizing.