Depends how old the van is, and what facilities the site has. Older vans can have rotting floors, especially in the shower/bath area, I would also beware of the electrics on older vans. If it is a purpose made static caravan then the electrics will all be 240v domestic, if it's a large ex towable van then the electrics will be a mix of mains, and 12v run through a rectifier of some sort, these need to be carefully checked out. Having said that, if you're on a nice site in the country, then it's a great life, we have friends who live in one down south, and on site vans there are costing as much as houses up here where I live. Get it surveyed first is really the answer. Then enjoy it.
Giok