The eccotemp L5 gas powered caravan heater is a good option. All you really need to add is a gas bottle,.. I think even the regulator is included with the heater.
If you want to run the heater with the van doors closed, you will need to put vents in the van roof, and make some kind of extractor hood.
One thing I would recommend is controlling the heater with a relay. It simply connects where the on/off switch is, and takes 2 wires from the pump. It simply switches the heater off when the pump stops,... no more worries about overheating, or wasting gas when the water isn't running. The heater has overheat protection anyway, but better to have this as a failsafe too.