Mine [2017 van] are bzp [bright zinc plated] and not chrome plated. They should not corrode and should not need treating with WD40 so I think you have been told a porky there. A couple of mine whilst not rusty are a little tarnished in appearance
What does confuse me is you mention the edges appear to be nickel plated. This could be several things. The edges are not as exposed and have discoloured at a different rate, the plating on the edges is a different thickness, or you are unusual and do have chrome plated hinges. I say this as to chrome plate you first go copper, then nickel before the final coating of chrome.

Whatever the case they should not be rusty and I would ask that they are replaced.

Colin

I've just had to replace one for the first time, on our van. Mind you, it is 20 years old.

Randa

I don't believe they are a one off for Hymer. Try here, 3rd from bottom.
https://www.blum.org.uk/shop/cabinet-hin...t-doors-865094/

Colin

Hart will not deal directly with you are they are a strictly trade only supplier with £25 minimum order.
Best bet is to e-mail Blum https://www.blum.com/gb/en/ with the part number [and description as Hart may re-number the product] and ask for a schematic of the hinge. I think you will get most of the sizes you need from the small schematic down the right hand side of the link that examples a 40mm hinge.

Colin