When is a new RV not new?

I can understand your concern ... it just doesn't seem quite as new, as untouched, as you thought it would be, especially considering the $$ you must have paid. Having said that, the only reason you're aware of this is because you're the one who noticed the problem. You could insist on getting another unit without knowing that they repaired a bunch of things before you even test drove it.
If it had been in a crash, that would concern me, even if it was repaired by MB. I think if it were me, I would make sure the differential is new and then move on to having fun with my new toy.