In my experience cleaning like you would the rest of the van is enough. In my case I wash the van (and camera / lens) with one of those extendable brushes with long, soft, bristles. Anything better isn't visible when the camera is in use.
Paul took ours off a couple of days ago as they packed up on our recent whistlestop tour of mid Wales. Although we've only had the MH less than a month and it's still under warranty, he knows he's more capable of fixing them than the dealer from where we bought it and we didn't want the MH to be sitting in their yard for weeks while they fail to discover the fault.
They’re now fixed (wiring fault) and refitted.
The reversing camera on my car has a wash system just like your windscreen. When the car is in reverse and you wash the back screen a separate washer does the camera. The camera is only a couple feet of the ground and I never even look at it, I had the car 6 months before I even knew it had a washer! I wish my van had the same thing.