On the Carver 2 water heater, there is a non-return valve. As you look at the back of the heater from inside the caravan, it is situated near the floor on the right hand side.
The white plastic from which it is manufactured turns brittle after a few years and, if anything in the locker bangs against it, or the hose attached to it, it breaks. They are replaceable. it simply unscrews from the the heater unit and fitting the new one is simply the reverse process. If it is cracked, a new valve should be available on line. Unfortunately, my instruction and part no. book went with the caravan to which the Carver 2 was fitted. If you go on line, look for an exploded diagram of the unit showing the components. The valve will be shown on the diagram. Obviously, draining down is a must before disconnecting anything! As Tommo says, make sure all the water connections are tight and make sure that the jubilee clips are in good order. Make sure that none of the water pipes are split.
Make absolutely sure that the electric heater element is switched off at the mains. If it is switched on with no water in the heater unit, it will burn out and you will be looking at a bill for about £50 for the replacement unit plus the cost of fitting it if you cannot do it yourself.
I am not saying that that is the problem as fault finding at long range is fraught with difficulties. It is worth checking.