There seems to be some confusion between Park homes and static caravans.
Park homes are residential, 365 days a year, you don’t have to leave them at all.
Static caravans are holiday homes, and most holiday home sites close on 30th October and reopen on 1st March (some vary and only close for a month, it depends on their licence). You can stay in your caravan for the entire time the site is open if you wish, but you have, by law, to vacate when the site closes.
Post will be delivered to individual park homes, just like any other permanent address, but static caravan sites usually have an office where all post is delivered to and owners of statics go there to collect their post.
As to whether or not park homes are a good idea, it really depends on a number of things. And most especially doing your homework about all costs involved. Most park home sites are totally legitimate, some aren’t.