Using the vans battery works if you do either “quite a lot of mileage” or “not a lot of work”.
I had a vw caddy as my first van, and for the first 6 months i ran direct from vans battery. No problems until one day I jumped in the van at the end of the day and was stranded.
The problem is that you will always have enough battery to power your pump, but not always enough to start your engine.
Now I’ve got a hot water system I have no choice but run leisure batteries in the back.
You can make it easier for charging now though, I’ve got my charger (CTEK) permanently connected up to the batteries, so I just run an extension cable out to the side door of the van and plug in. Takes seconds.