As far as I am aware, from April 2013, caravans which exceed the limits of size of a trailer for the time being permitted to be towed on roads by a motor vehicle having a maximum gross weight of 3,500 kilogrammes and is either:
a. manufactured to standard BS 3632:2005 approved by the British Standards Institution, or
b. second hand and manufactured to a previous version of standard BS 3632 approved by that Institution and were occupied before 6 April 2013.Will be zero rated.
As a general point VAT should be charged on taxable supplies by a taxable persons. To check if he is a taxable person, ask for his VAT registration number then check it with HMRC. If he is charging VAT I am assuming it is for a supply other than the caravan (i.e. services associated with the sale). If so, what sort of service has he supplied as this will impact whether VAT is due? If this commission is indeed payable for the supply of the caravan (which is a question of fact) then the VAT charged should be at 0%.
Hope this helps
Basically I am asking what was the "commission" for? What does the money you are paying relate to? Are you contractually obliged to give them the 15% of the sale?