We did it for a few Months in a Toyota Hiace Van, it was when we were younger. This was one with just a mattress in the back and everything else we carried. Food stored in cardboard boxes, water stored in several 2 litre plastic bottles and an esky in the back. It was rough and we had little money but the adventure was more important than the comfort.
The worst thing was the flies and mosquitoes. Distinctly remember stopping for lunch one day up in North Western Australia whilst touring. Needed to stretch our legs and was looking forward to a nice tuna sandwich (tuna out of a can). As soon as we opened the door literally thousands of flies swarmed to my wife and I, on our back, on our Month, trying to get up our nostrils. It was so bad we had to get back in the van and close all the windows and have our lunch right where we sat all day. Then night comes along and it's like a change of shift, the flies knock off and the mosquitoes clock on.
If you really do it cheap then you need to stay in rest areas in remote areas where there are no fees. This is ok but you will be lucky if you have a toilet let alone water. My wife hated using the bush as a toilet but we persevered as the point of the exercise was to see Australia. Every second or 3rd day we would hit a caravan park so we could do the chores, laundry, filling water bottles, showering and cooking. Every night at a rest area was $$$ saved. Stayed in some amazing bush camps. One time we were driving in the Northern Territory and saw a sign saying, "Disused WW11 Airfield" so I thought, lets go have a look. Drove down this overgrown gravel road and sure enough there was this old airfield. We ended up camping there the night, no one about.
In my opinion you got to be sure of the rig you will be using and living in. We did it totally with nothing, just a van. It was hard going. But if you do it with a $200,000 RV with satellite, toilet, shower and heaps of water [AND FLYSCREENS, ha] it would be much more bearable. We still remember those days well and saw parts of Australia very few people do and are happy we did it. I just recommend being prepared for everything and I mean everything.