Jim I have used a hot box to provide hot water for my carpet cleaning for the last 10yrs, it is hooked up to my truckmount.
the biggest problem with them is they are too powerful they are designed for machines that have a flow of around 15lt-18lts so controlling the temp is very tricky.... you get massive heat spikes where pure steam comes out of the wand until the thermostat kicks in.
I have a bypass fitted so half the heated water returns to the fresh water tank, this helps control the heat better.
you can only get diesel hotboxes.