Water Heater not working

Don’t say which model you have fitted but I believe same element is used for both tasks, so if heating is on,water should also heat up IF boiler is full of water. Water would heat much quicker without heating saping away the heat.
May take some time to heat water with heating on. 
Are you sure hot tank is full and have you allowed sufficient time for water to heat, also did the dealer test all ok after chargeing fuse ?

If the hot water has a 'boost' setting use it.  It will temporarily stop the Combi heating the motorhome and so concentrate the heat on the hot water water tank.

The OP has said 'no hot water'.  Does he mean when the hot tap is opened no water flows, or, water flows but it isn't hot?  I have assumed the latter.  If the dump valve is open the water pump will run continuously whether the taps are open or not.  If he hasn't filled the hot water tank fully, every time he opens a hot water tap it will fill a bit more until it is eventually full and water will flow.

Bill ??