3 years ago I looked at self build fitting a hot water system with an Eberspacher diesel water heater at its heart, with, plan a) a small hot water cylinder, and plan b) an alternative instant / constant flow system using a heat exchanger (have a look at Jerba camper vans, and also, if I recall, Australian sourced plate heat exchangers) but the ancillary parts took up too much space inside a 'sleep 4' carry 5, lwb T5 sized vehicle, hanging more domestic water equipment underneath the van compromises all season use at freezing and below.
The VW California lends little simple redesign scope, Pint pot and all that.
Good luck with your research.
Rob H.