I've only ever heard of 2800 watt Onan generators permanently installed in gas engine Class B's. Some B van owners carry a portable generator.
Do you need it to run the Air Conditioner?
If you can post a photo of the outside cover for the water heater I might be able to tell you what brand it is.
Suburban water heaters are/were quite common in B vans and are typically 6 gallon.
Airxcel | Suburban Manufacturing Products They can be pilot light or direct spark ignition (DSI). Some models also have an electric heating element.