a couple of years ago i looked a a small propane water heater from a WFP company in crawley London ( Omnipole i think) at the time i did'nt have the knowledge to adapt this for carpet cleaning use, but now i know it could be done.
cost about 35o pounds, but the problem at the time was it had no thermostat so when it was on it was constantly heating water, now I couls fix this problem with a bypass vavle.
might be worth looking into, it was also small about a ft wide without the propane bottle.
Love from egypt

Mike