We had a 33", three bedroom static for over 20 years - and loved it when the kids were young, but grew out of it as they got older and wanted to spend weekends doing their own stuff.
On the site we were on (in the grounds of a mediaeval castle), letting wasn't permitted for most of the time were there.
But, in the 80's, when it was allowed, there were lots of issues with the renters - bad behaviour, didn't look after the caravans, didn't follow the site rules; basically didn't care because they were only ever there for a week.
A lot of the owners who rented out regretted it big time - repair costs, cleaning/replacing carpets, furniture, crockery etc, plus, it's a bind having to wash all of the bedding, all of the time - on top of washing all your home stuff - more of an issue if the site is not on your doorstep.
Personally, I wouldn't say it's a gold-mine business opportunity - if after all these years it was, then a lot more individuals would be doing it.
On reputable sites, there is also a bit of class discrimination, with renters and rental owners sitting firmly in the "undesirable" class amongst the 2nd/holiday home owners, which can also make life/things a little bit awkward.
But, I suppose it all depends on how the site owners run things.