Dear Tim.
I am a motorhome owner & heard about your issues on an internet Forum (Wild Camping). I understand that rows of vans parked in prime spots for days on end are not an attractive proposition, however I would like to remind you that these people need to eat, buy food & fuel etc. Motorhomes are not cheap to buy, rent or run, so mostly the people in them will be relatively prosperous. They are attracted to your town like any other visitor by your many attractions & will want to sample them, we do not live soley on tins so they will be using local shops, pubs, restaurants, leisure attractions & garages.
As with all car parking problems, I believe you need to maximise throughput & limit long stay, low spend visitors in favour of people who stop a short time & enjoy local shopping. Perhaps something along the lines of parking meters could be used, 50p/hr with a max of 12 hours would allow people to stay either a day or a night, but not both.
Your Motorhome dedicated car parks along the lines of the French Aires are an excellent idea, but you need to attract visitors, not repel them. £10/night is higher than is normally charged for a small Certified site with water, toilet disposal & possibly an electric hook-up. So why would anyone prefer a carpark with absolutely no facilities? I wouldn't pay to go there. But I might use it if it was free. The benefits to you of making it free are that you will get more viisitors to spend their money & you will significantly reduce the costs of policing it. Your proposed £10 cahrge will generate very little income as few vans will pay that for a stay in a car park AND you will need to pay someone to police it.
Probable income - say 2 vans a night for 50 nights a year (about £1000)
Probable cost of policing 6 months salary at min wage (ignoring the staff overheads!) = £6000
You will need 6 vans a night for 100 nights just to cover the basic salary cost, surely even you can see that that is hopelessly optimistic at such a high price.
Good luck with your experiment, however it turns out, Steve Kean, Motorhome (& boat) owner.