Looks like an instant hot water heater.
It's just about the biggest coach built they mounted on a Commer/Dodge chassis so they do struggle a bit - loads of room when you set up camp though
I still have what is left of one of these. I still have a lot of parts from inside and all the plastics from round the lower part that I had repaired ready to refit.
There were a few nasty incidents with that type of heater, in particular the Ascot type. An old work colleague of mine died because of a faulty one in an apartment he was staying in while working abroad.