I think it will depend on the size of the tent. If its a pup tent then some charge and some don't.
If its a larger tent then you could have problems, as the site may see it as two units on the same pitch and that may contravene regulations. (there was an item on the forum where a caravan was being towed by a small motor caravan with persons sleeping in both. The site saw this as two units on same pitch and amongst other things fire regulations were quoted for non approval.) I realise that your tent will propably be no larger than an awning, and people can sleep in awnings, but rules is rules.