Thanks Lutz, towing a caravan through France down to Costa Dorada in Spain just wanted to settle a difference of opinion.You don't say which country you are referring to. If you mean France, only the height of the caravan makes a difference. Folding caravans under 2m are cheaper.
In Italy there is a 60% surcharge for twin axles but otherwise I don't know of any other country that differentiates between single and twin axles.