I think its a great idea . Weve known a few people who for a various reasons have chosen to live in Tipi's Yurts 'Tree house ' Caravan ' Bus ' Narrow Boat ' Old Tram ' and a Railway Carriage . finding a site is your main issue but a campsite is probably not your best option for a long term stay .
Out of choice we lived in our static caravan for nearly 2 years, absolutely loved it. Good ones like the one we had have central heating double glazing washing machines baths etc. The park we were on had a few people who rented them for the whole season (11 months). If your interested in going down this route I would suggest you find the park you like and just walk round and ask some of the owners if they know anyone on site whos interested in renting out for the whole season or go on gumtree and look for private rentals, then look how much the site fees are,then if any come up base your offer on that but remember owners are looking to make a profit. We found one for our daughter and partner for 2 months while they saved for adeposit the people wanted £250- £350 a week. We offered £150 a week and they accepted. That's with gas & electric all in and don't forget theres no council tax on caravans.
I saw some on ebay for between 3 and 5 grand,i'm sure holly bank caravan park,rixton cheshire,0161 775 2842 has people living there all year round,I know it deffo used to ,hope this helps x ps it takes tents too