Hello barny
Can you let us know what type of fresh water system you have?
Do you have an external supply from a container or is there an internal tank? If it's the latter, how do you fill it?
As far as the water heater is concerned, I think that the following will apply whichever way you get your fresh water:
There are normally two switches which control the heater, and most often they are mounted together on the side of the wardrobe. One is the switch for the fused spur to the water heater switch itself - this has a small rectangular symbol with a vertical line through it and an illuminated LED. This needs to be turned on to be able to switch the heater on. It's there to isolate the heater (and its switch) from the 230v mains.
Adjacent to the spur switch there should be an illuminated rocker switch, and this is the one that turns the heater on.
Before you switch either of those on, you need to fill the heater's built-in tank.
Assuming for the moment that you have water coming out of the taps when you turn them on, turn both the kitchen and bathroom taps to hot. This purges the air from the hot water system and fills the heater's tank. When both taps have stopped spluttering and run steadily, you can shut the taps off and turn on the heater; spur switch first and then the heater itself. After a little while this will give you hot water. Its temperature is controlled by the thermostat in the heater. The heater tank is pretty tiddly, and as you use the hot water it will be replenished from the fresh supply.
Do check that the water system drain valves (usually under the seats in one of the rear quarters) are closed, otherwise your fresh water will just dribble out onto the ground.
As far as the waste water is concerned, what happens if you pour some down a plughole? Does it come out of a grey corrugated pipe underneath the van, or does it go into a tank under one of the rear seats?
If you fill us in on those missing bits of information we can get you sorted.
.