Some French campsites charge double for double axle a few ban them. In UK same price single or double. Bear in mind most Hobby twin axles are not tourers as body length is over 7mtres so can only be legally towed by vehicle over 3.5 tonne gross weight.
Usually the same, I can only think of 2 sites we have been to that charge £1 more. Never been charged more abroad but we avoid municipals who often don't accept t/as.