Hi
To my knowledge (and I am happy to be educated if this is wrong), most steam generators are run on either diesel or paraffin, but they still need an electricity source (mine is a small generator) to run.
Cleaning garage forecourts (I assume you are referring to petrol stations) is something I was asked to look into by a customer and I did some quite in depth research into the rules and regulations etc.
It is an absolute minefield if you do it properly so much so that I decided to walk away.
The main problem was in fact the one youo are faced with... you are not allowed to have the pressure gear on the forecourt due to the ignition risks, and even if you can run enough hose from 'off site' the risk assessments etc are a nightmare.
Good luck with this one, hope you have more luck than I did.
Andy