We're going to a site which charges £7.50 per adult per night low season for tents, but £14 for a caravan and 2 adults (but it's extra for a caravan awning). It's a brilliant site with excellent loos, showers washing up facilities, and even provides a fridge freezer! They also have a drying room for boots and wet gear, so I guess they think caravanners will use these facilities less, hence charge them less! I'm not sure whether we'll be charged as a tent or a caravan, though!
We have gone from a tent to a TT and not noticed any difference in the site fees. Our TT is smaller than the tent so if anything it ought to be cheaper!
I have found that some site try to put the larger Trailer tents onto caravan pitch, thus resulting in you having to pay the extra. But it is all down on how big the pitch is at the site.
We have gone from Tent to Trailer Tent and we have yet to find the difference in fees yet as they have been the same as a tent.
Although i have come across one came site that you have to pay extra for the awning of a TT even though its not an awning its your living area.We never did go there mind.