Use a caravan hot water system, provides endless amounts of hot water for showers etc, and a bottle of gas lasts me ages.
And it won't cost you no where near a grand.
I have to disagree with you there jeff (without me doing any proper research 
)
So please prove me wrong!! 
But........ I was under the impression that these caravan/motorhome systems have tanks about 10L-16L and use gas to heat them?
But...... Most of them take 15-20mins to heat up to 70 degrees and if your using it for a big job the cold water will be feeding the tank as you use it and will not get a chance to heat up fully???
I may be wrong??
It would be ok on a small house where you would only use 10-16L of water to clean it and drive 20 mins to the next job and by then it's heated up and ready to use again.
Or is there a caravan/motorhome 12v gas water heater that heats it instantly as you use it?
Hi Matt
The system in my caravan is a carver,(can't remember the model number) if I jump in the shower it takes seconds to heat up, I can spend 20 minutes in there, and then the boss jumps in (not at the same time I might add
there's no room
) I can tell you, it stays hot all the time without any cold spots.
My caravan is stored about 6 miles away, so if I remember next week, I'll pop in and get the model number, but the more modern ones are even better (i believe)
And to top it all I think the one I have, is only a 9ltr one? again this I will have to check,
but No way on this earth does mine take 15-20 minutes to warm up to a decent temprature, it is instant.
If you have the time, do some research and you should find even better ones than I have.
You would not be able to travel to your next Job while it heats up, I think its Illegal to travel with the gas burner switched on and in operation, you can't even have the pilot light alight while travelling and this is why you switch to battery when travelling.