I think it's reasonable, after all they are liable for the van while you are not there and their insurance would have to cover any claim you made against them, which would put their premiums up.
Why not look for a site with in the same area which will tow your van off pitch to storage and charge you £1 a night then you ring them on Thursday and they will put van on pitch Friday afternoon and charge normal pitch charge for that time of season.
And if your lucky it might have a club on site
Seasonal pitches work out cheaper as owner is getting paid a higher price.every week than a empty pitch wether your there or not but no site is fully booked every day of the season usually busy when kids off school. But of course the seasonal vanner getting cheaper pitch price all year round the same pitch which they garden and decorate especially if they plan to stay for many years but cheaper than a static van or mobile home