Hello Simon,
Unless the regs have changed recently, to work on a private touring caravan where gas is not supplied by pipes, the requirement does not call call for certification of the fitter, However the rest of regs do apply and that does call for the person to be "competent".
It is s subtle difference, and I do not know why or how the anomaly came about.
It is in my view silly to make touring caravans a special case,as they more than virtually any other installation is subject to vibration and other environmental conditions that can easily deteriorate the mechanics of the system.
The full regulations apply to all other 'caravans' and mobile homes, and to touring caravans owned by a business or rented out.