I think it's a fantastic idea, more and more people go clamping now, but I would suggest doing a blowdry and hair up course too, target campsites with limited shower facilities that way you will get more chance of people coming to you after showers with wet hair for it dried, even £5 a blowdry, when your times are good would be £20 a hour on best case scenario.
If the interior was inviting, warm an well cmdecorated I wouldn't be put off when camping.
An idea would be have a gazebo by the door for a waiting area with seats.
Lastly, for the people on here who wouldn't want to be in a van, would an old fashioned interior campervan be possible to get hold of, that way you can strip te interior, section it off to have a indoor waiting area, a treatment area and a toilet, plus they have heating so great for winter xoxo