I did a calculation back in 2013 when the owner of a small site complained about people staying only one night and filling up with water. It seemed to me that on domestic water rates, a 100 litre fill-up would cost the owner 11p.If the public toilets are free, or a very small cost compared to the charge for using an elsan, inevitably many will realise that and use the toilets to empty their cassette, as probably already happens now without the elsan point.
There probably needs to be a moderate charge simply for any basic maintenance and that sooner or later there would be some cleaning. And these days water is usually metered, so anyone filling their MH tanks is incurring a cost to the facility provider. I do think many will baulk at a £5 charge, you see too many posts on here that £20 for an overnight stay on a site including EHU is excessive. Any charge made will need to be sufficient that it makes the cost of collection alone worthwhile.
Domestic water in Scotland is not metered anyway, at least it wasn't when I lived in Selkirk, I don't know how much Hawick council would pay for it. ISTM that it is not so much a question of the cost of the water itself (which never seemed to be in short supply in Scotland), more a question of amortising the cost of installation and maintenance, and a little profit to make it worth the effort.