Heres a way to establish if there's a pipe work problem with the Cascade heater.
Just turn the cold tap on enough to start the pump. Then if you open the hot tap fully the water should flow through the heater and to the tap.
If there is no reasonable flow under those conditions, then it points to blocked pipework.
However, you tell us its been 3 month since it worked. Did you completely drain the water system down whilst the caravan wasn't being used? If not its quite possible the heaters non return valve has been damaged by frost. The action of ice as it expands can dislodge the NRV's O'ring seal causing the valve to jam mostly shut. Some times the valve can be repaired by dismantling and reassembling it, otherwise it might need to be replaced.
It is vitally important to drain a caravan down if its going to be left unused during frosty weather.