Hello Russell,
I assume you have a Carver Cascade 2 or 2GE water heater. It sounds as though the cold water inlet non-return valve on the heater has jammed. This can happen in the heater is not fully drained and there is sufficient frost to freeze the water in the valve body.
The valve is located at the bottom of the main casting on the left hand side inside the caravan. It screws into the casting, and can be replaced.
As the water freezes, it expands. Any water in the valve body will freeze and push the internal 'O' ring of it's seat and damage it, and jams the valve .
This severely restricts or totally prevents the flow of cold water into the heater, and so you will not get any appreciable amount of hot water out.
Do not use the heater until the valve is sorted out, as you may overheat the heat exchanger and damage the fusible plug.
To prevent re-occurance of the problem always ensure the water system and the heater are fully drained if the caravan is not in use and there is a danger of frost.