Lee, it depends what you want to do, it is safe to leave on, but some people prefer to switch everything off if leaving the van for any long period of time.
Both heat sources are thermostatically controlled, so will switch in and out as required.
If you forget to switch it on whn you return to the van, you have at least a 40 minute wait for hot water,,maybe inconvenient, but not life threatening!!(unless you havent done the washing up!!)