There is a publication called "camps"
http://www.campsaustraliawide.com/
it is updated every couple of years and lists all the free and cheap camping spots around the country
for our travels with or without van in tow its our bible
we pick a destination and plan a route based on distance and the availability of public access areas to stop
it also has great detailed maps
a great tool to help reduce costs
WRT seasons
its north in the winter and south in summer
of course this is when everyone else travels and can get a tad busy, for example last year we were going to do the gib river road until it was described as a "carpark" by some one who had just come off it