I don’t have the same van as you but I know that it is sometimes possible to take a model number off the rear light cluster and look it up in the Hella catalogue on line. Failing that O’Leary Motorhome breakers can have parts. Or if you need to buy a new part it is possible that E S Hartley Motorhomes may even have a spare in stock, if not they will order from Hobby in Germany.
A few years ago I needed a rear light and Hartley wanted £285 with an 8 week delivery time from Germany. Fortunately the light started working again so I didn’t proceed. I would imagine the cost is now higher and much longer delivery times.
Good luck sorting it out and let us know how you get on.