Most car parks where aires are situated are large enough to accommodate both cars and vans, never full. Hawick for example has 400 car spaces and the car park has just been extended at the bottom for motorhome use. Nearby Jedburgh has 3 car parks in town and these are never all full at the same time. As Pugsy said we hand in our receipts wherever we go. Usually send a thankyou email to the towns as well attaching email receipts. If most people do this it helps towns see how much they can benefit.Interesting arguments about the economics of this whole issue - "we bring revenue to the area" is a common one and undoubtedly must have a grain of truth assuming that people do actually get out and go an eat/drink/shop locally rather than sit all day admiring the view and watching TV.
That said, I am not sure how this value can be calculated. A Moho with two occupants filling a parking space for a day denies that space to vehicles that could be carrying four or five people - people who might otherwise make up important passing trade to the locality. Not an easy equation to work out methinks . . . . . .