Hot water hot when heating is on is this righ

Everything said is correct.  I would just add that if you want hot water its still best to turn the hot water on.  If not the water will heat up more slowly because a) when the central heating fan is running most of the Truma's heat will be used to heat up the van rather than the water and b) as Bargee has said when the van reaches the temperature set on the Truma's control panel, the heater will cut out so no heat will go into the water.  If the central heating is on and hot water is needed in a hurry, its best to use the 'boost' function which temporarily gives priority to heating the water over heating the van.