Whale Electric Water Heater ...1500W not working

Hi, i own a 3 year old Motorhome and have a Whale 8L, Electric and Gas Underfloor water heater installed. The water heater works on gas fine and on 750 w electric option (1 wave), but does not work on the 1500w option (2 wave) anymore......all fuses and circuit breakers are fine, and there is no error showing up on the Whale Duo Control Panel. Has anyone encounter a similar problem?.....and if so how did you resolve?

Thanks Graham