As Mike Tyson once said..... everyone's got a plan until they get punched in the face.......
Aka in window cleaning terms.....
"Everything is fine and dandy until your van catches fire or explodes."
Using and operating an outdoor shower as an water heater is fine, but just don't expect your insurance to pay out if there's an accident.
I'm still yet to see it in writing from anyone that they are covered to Operate a gas boiler inside their vehicle while it is unattended.
The amount of window cleaners popping up now and setting these boilers up it really is only a matter of time until there's an accident. This is why I went down the immersion route.
That and also one day I didn't tighten up the gas inlet on the boiler enough, which meant gas was leaking, I ignited the boiler to start work and then then the naked flame caught some of the gas and ignited. I've never used one since.
We can all be as vigilant as you like but in all honesty are you going to do all The needed safety checks every time you pull up to a job? Disconnect gas bottle, reconnect gas bottle, make sure hose is tight to boiler etc.
I know I wasn't.