How are Autoroute tolls checked for Motorhomes

I realise that there are different classifications based on Gross Vehicle Weight and length, but how do the people in the toll booths know whether your motorhome is under or over 3.5Tons? Is it a self selection based approach?

On auto tolls, if you select category 2, but you're in a category 3 vehicle, can they detect this?

Is there any form of ANPR that links to vehicle weight?

Do they have weigh bridges or other means of detecting vehicle weight at the toll booths?

Presumably there are fines for not paying the correct toll that outweigh any savings?