If you are space constrained you could consider:
1. LPG/120AC powered - Malaga water heater, it is a little smaller than NA brands and it doesn’t require cutting large hole on the side of the van, still hole but smaller.
2. Engine coolant/120AC powered – use marine water heater, popular for DIY folks is Isotemp. I have Isotemp heated by independent diesel furnace. Heated by engine coolant but keeps high temperature for at least ˝ day. Or use hot plate heat exchanger, it extracts heat from the running engine coolant.