Hot Water Heat Up Time - Webasto Dual Top RHA-100

Hi. We have just bought our first Campervan, a 10 year old Devon Provence, from a dealer. It is fitted with a Webasto Dual Top RHA-100 which uses diesel to heat the hot water and space heating hot air.  We were concerned that the hot water was not operating properly as I could not manage to get anything but luke warm water from the taps. The van has been returned to the dealer and he tells me there is nothing wrong and it should normally take 45 minutes from switch on to start producing "hot" water. Is 45 minutes normal? It seems an awfully long time. I actually left it on much longer than 45 minutes without getting any hot water. Any advice would be greatly appreciated. Thanks.