As DD, realistically what is the chances of these meters being so far out of calibration that you are being robbed, or equally even getting a bit more than you are charged for?
They will on a site be exposed to checking more frequently than your home unit, there a minimum of every 10 years, then its not recalibrated but changed out.
A site owner over charging I suspect unless by a trivial amount is exposing themselves to be caught out, the serious ramifications of that ought to make doing so pretty uncommon, IMO.
As I said earlier, I think the rate should be on the notice board along with any other, "extras".
Ideally they ought publish it on booking forms, so if it is the decision maker to use that site or not, those users living in fear of a pending rip off, can avoid it.
Add into the mix; IMO if sites at the lower pricing and margin end of the system like many 5 van sites are, simply will not be able to risk keeping open during the winter unless they do meter EHU usage.