What water heater is fitted, if it is an Ultrastore then as long as the mains power is available, and the unit is switched on( AFTER filling with water)then it should work as it a direct feed from the distribution box.
If it does not work at all on electric, then most likely is the heater element has failed and requires replacing.
If you had left it ON , or switched it on before filling with watr then the overheat cutout has worked and will not reset unless the power cable is removed from the van for 15 minutes.