Hello Michael
The situation regarding the maintenance of manufactures warranties is a grey area.
The manufacture has no obligation to offer any warranty to the end user, it is in fact a gift, and to a large extent the manufacture can set criteria to limit the gift and is in addition to your statutory rights.
Incidentally you statutory rights are a contractual obligation between your self and the seller (as differentiated to the manufacturer of the item).
You must check with the manufacturer to see if a variation is acceptable. i.e. service work carried out by a non franchised workshop.
In a recent challenge, a similar requirement placed in car warranties was deemed to be unfair, and provide the service work was carried out in accordance with the manufacture s schedule and using genuine parts and to the same standard, then work could be carried out by any competent workshop. - not necessarily affiliated to the manufacture. However the same may not be true of caravan warranties.
Check with the manufacture.