Officially its for vehicles 3500kg and under and also 3m and under. We used our Liberte tag for years in our first 2 motorhomes (one being 3.1m tall and the other 5t) with no problem and have used the B&P tag in France, Spain and Italy now, again with no problem and we are 5t and a shade over 3m tall.
AFAIK there is no facility to weigh MH's and tbh I really don't think anyone cares as long as they are picking up a fee. It's small beer given the amount of usage of the tolled roads abroad.
I 'think' that there is some Tag system available for larger vehicles, like commercial ones however I'm not familiar with it. I am aware that the difference between Class 2 and Class 3 tolls are big, very big hence when in pre-Tag days I would press the button and pronounce in my very worst Franglais "Je suis un camping car. Class deux svp..." if ever it came up with Class 3