I agree with Andy on this one. If you've ever worked in a factory with propane run forktrucks, it's a nausious smell. as well as this, i would'nt trust an internal combustion engine indoors in this type of machine.
Health and safety, A hot exhaust and indoor air quality, a pressure valve fails spraying hot pressurised solution around the interior and imagine a fuel leak and source of ignition.
You would still physically need to move it into the building, but also you would also need to carry the pipes from the vehicle as well. I would rather run pipes out, presuming they can reach the destination.
I think this type of machine does have a place somewhere, but for someone who employs their own cleaners, and the machine is left on site.
Dave