Not knowing the context of the question I can only guess. To be a home in the legal sense I think it would need an address, it would also attract council tax. Which depending where you live could be reduced if it is a second home.
Got a friend visiting at the moment who retired from the Benefits Agency a few months ago. In her opinion (unofficial obviously) a second home would probably disqualify you from certain benefits, but it is in the definition. A static caravan is "fixed" as such and could be classed as a second home but a motorhome, like a caravan, is mobile and is most likely to be looked on as a vehicle and ought not to be thus considered.
But as she has always said, the rules seemed to change from week to week!