Did this with a mate in Australia a few years back. Bought an old flat 6 Holden Kingswood estate and bunged an old foam mattress in the back. Hand made curtains (old material and staples hanging off string), rigged up some ropes to hang surfboards off the ceilng at night so they didn't get nicked.
We kipped in it for about 3 months, so no campsite fees (carparks, roadside), washed in public toilets and the sea :-), Lauderette every couple of weeks. Spent about £5 a day on food and drink, jobs a good'un.
But- it broke down loads so go for aa cover or something (i think we called out their equivelent about 5 times).
Sold it on for a small loss afterwards, but more than made that back in saved accomodation.