We used to do that when we ran an ETM back in the day. We had a propane tank mounted on a dolly with a propane bottle beneath it for hot water. Worked well but this was in the early 80's, wouldn't do it now with all the sensors and alarms. We also used to pull our HydroGenie into the mall and clean since it ran on propane back then. Wouldn't do it now.
Yes we do it a lot, but check your own state's regulations.
Just make sure you have a heat deflector cap on the chimney and that you don't position it right under a sensor.
You will find that if the building has been built or retrofitted in the last 20 years, it will likely have combustion sensors that preclude the use of an open flame indoors. As well, most fire regulations in metropolitan areas forbid it, not to mention the liabilities. These very details are what killed the manufacturing of the Steam Genie Electro-Genie (equipped with an LG heater) in the early 90's.