Hi Graham,
Time to get your Bankcard out and get a Forum subscription! David
@wildebus is fitting my Polarmax Battery next week, and he, plus other members such as Merl and Phil Admin have a huge amount of knowledge and practical experience of motorhome electrics in particular, and of conversions/adaptations in general. You will more than recoup your subscription cost with the first piece of technical guidance, especially if you compare the price/delay that getting a Dealership to sort out the same problem would entail. Full membership also allows you to attend Rallies and gatherings
Steve