Definitely not something powered solely of the leisure battery.
You really have 2 options, both that work with little electrical input.
Gas / Diesel
The blown type of heater, like propex or Eberspacher which use gas or diesel. They burn the flame and vent all monoxide/fumes to the exterior via a small exhaust and blow the warm air in via a heat exchanger. Very safe and has the benefit of models having timers and thermostats...... but expensive to buy.
A fixed gas powered caravan heater, specifically designed for use as all fumes are vented to the exterior. No naked flames are burnt to the vehicle interior. Safe and quite efficient, also available widely secondhand and relatively inexpensive. Use no power at all to run except for the ignition which is usually manual anyway. Downside is that they are quite big and need quite a large hole made in the floor to accommodate the unit so may not be a runner in a van conversion. This one is cheap,
Caravan Heater on e-bay, but you need to get someone knowledgeable to evaluate if it can be fitted and you have room for it. It is also the floor vented model so easy to fit as no ducting is required for a flue.
There is mixed views on using a naked flame heaters in vans (heaters not designed to be vented to the exterior).... for what’s its worth I wouldn’t reccomend their use in a van because of the higher risk of carbon monoxide fumes and also for safety reasons.
Whatever way you go I think a CO detector is required.
I have fitted a few of these to conversions, they work well and are efficient.
You can get one that integrates heating and water heating too but are expensive.
This one from ebay is relatively good value and work really well.
See
Propex heater on e-bay
