I wonder how much of the problem of inappropriate use of parking areas, that results in barriers and signs going up, is down to people hiring vans/MHs? These people are unlikely to be on forums like this or aware of what might be termed "etiquette", and, as has also been mentioned, be more likely to holiday abroad once things return to something more like "normal". They probably don't realise the after-effects of their behaviour, nor care if they're unlikely to do it again? Does there need to be some effort to educate these people, perhaps via the MH hire companies? Do they have an association or some such?
I’m sure that some of the problem is due to newbie motorhome owners and hirers, due to the ignorance of the etiquette.
Plus, knowing some in the industry as I do, I strongly suspect it’s partly the fault of some hiring companies, and sales companies, who will say anything,
anything, to make the sale. “Sure, park where you want, any time anywhere”.
Plus, the regular motorhomers who for many years have taken advantage of lax enforcement. The ones who said “I know the sign says no overnight camping and is legal but I’ve always got away with it”.
Will it return to some normality in the next few years. Probably, as holidays go back to ‘normal’. But it might be too late.
Sadly I don’t think your suggestion of using the hire companies to spread the message will work, as it goes against what they want (short term). I did think that perhaps all wild camping organisations could formally collate information; this was met with mixed response. But again, having been actively involved with campaigning for nearly 50 years (with subjects much bigger than parking to be honest) I know how difficult it is to get people involved. Even those of us committed to a particular cause sometimes struggle.
Re that particular sign, I would interpret it as saying:
1) no overnight camping in the car park, which may or may not be legal
2) please don’t camp in lay-bys, a request only
3) general info that you need the landowners permission, added to make it all sound official.
I think it was Robmac who suggested avoiding the car park and parking discreetly elsewhere, and that may be the best idea for now.