Hi,thanks for that the big grey box in photo is the existing internal battery box, so the big carver heater in the new photo just heats up the water,is that right and does this hold water or just take it through from the aquaroll as it needs
Photo 3 is as said, your battery box with mains lead coming into the van from it, and what looks like the 2 wires to that external pump connection disappearing into it.
Your new photo 2 shows the water heater tank on the right, in the centre is your existing cold water inlet, and the power for the existing pump. You must have a pump that plugs into that on the outside of the van. The thing on the left is a surge damper to smooth out the water flow.
The cold water supplies your cold taps, and fills the water heater tank, it holds about 9 litres. The water is heated in it by gas and/or electricity, depending on what is fitted, then supplies your hot taps.
It is not an "instantaneous" heater, it's only about 800 watts.