Are you looking at hiring a car? What sort of set up are you looking for, basic, family etc. What area's are you hoping to visit or things to see? Are you wanting the convienince and facilities of Caravan Parks or more basic and bush sites?
You are heading into WA around the time of the school holidays, so may struggle to get into Caravan Parks, I know for Christmas, New Year and the school holidays they get full pretty quick. If you don't need power then you have more of a chance.
That time of year you are also probably better heading south of Perth, go via Mandurah (fantastic town on the estuary, boat hire, dolphins - the foreshore), Bunbury & Bussleton (on Geographe bay - more Dolphins, the longest Jetty in the Southern Hemisphere), Dunsborough, Yallingup & Margaret River (part of the Margaret River wine region, great surf, loads of wineries, great food & loads to do), Pemberton (Karri forrests, train), then on to the South Coast - Walpole & Denmark (tree top walk) Albany & Esperance (great history, whaling - not now). Have a look here - http://www.australiassouthwest.com/en/P ... fault.aspx
Not so much to do if you head North but some fantastic scenery and it will be getting hot.
If you fly into Perth the closest Camping shops for one hit are BCF in Midland and Myaree or Ranger in Bentley or Canning Vale. You are also fairly close to the Swan Valley which is worth a couple of nights to visit.
Just let us know if you want anymore info on sites or places to visit