Immersion user here - in van, not static.
650L layflat tank. Installed sideways across van behind cab bulkhead.
Immersion (3kw/240 volt) bottom left rear of tank (looking in from rear doors.)
Dedicated old shurflo pump and battery and charger for circulating water to avoid hot/cold spots.
Timer for immersion.
Cold day forecast, less than 6 degrees I want supple hoses. Put timer on for 4 hours overnight, no recirculation used.
Colder day but above zero. Timer on for 6 hours, recirculation may be used
Colder again zero to minus 3, leave on all night use recirculation.
Colder than minus 3 - may work if dry.
I have no experience of gas/diesel water heater but have a hot tap from my house boiler outside which if I am doing a filthy and large all day first clean like a school (rare) I fill one of my tanks with hot tap water (40 degrees) and use it for the first wash and then the second van with pure to finish.
If I did this more then I would deffo invest in a hot water heater on board.