Hi Craig, you got me thinking, about that so I logged onto my account and found this info.
I’ve got a caravan... can I have a tag?
France - Liber-t tag
Yes! Provided your vehicle and caravan overall height is less than 3 metres and GVW of the towing vehicle does not exceeding 3.5 tonnes. You can use any non height restricted lane that has the Liber-t symbol illuminated.
So the tow car has to be under 3.5 tonnes. They also send you a reminder when your tag gets to 5 years old and needs to be replaced.