If you book as trailer tent or camper you usually have the option for leccy hook up and often have hard standing. If yer happy with no leccy and purely grass then just book it as normal tent.
I bought a caranex to reserve my pitch when I'm off out. My tent doesn't have the option of the additional room so this was the next best thing I could think of. I had thought about chucking down a cheap pop up tent until then.
We use a post with number plate, and generally leave a windbreak/table/chairs on the pitch .. not with a rooftent, but when we've used a motorhome which is effectively very similar!
I normally just book under a tent and car via campsite websites and if it's over the phone I will explain what a roof tent is. A roof tent on a car probably uses up less space than a ground tent with a car, so maybe as their popularity grows there might be a different/cheaper rate?? (I wish!)
We do this a fair bit - generally either speak to them on the phone and explain what it is or just book a camper van space. A ground tent also works to reserve you're spot