I have had an ETM for 34 years, way before there was an ETM.
Back in the day I had 2 generators in the van, on a stacked shelf, at the side door of the van. Both were 5K sets, so they provided plenty of juice to run 2 separate vacuum systems and a Cat 290 pump, and an LG 3HT for heat. I used 2 steel drums for waste tanks and cleaned quite successfully for years. Along the way I also purchased a gasoline truckmount from Cleaning Technologies, which I still use today after some heavy modifications to run dual wands (now in its third van).
I have had my current ETM for just under 20 years. I use a Honda 10,000 watt genset in the van, with a fan directed to the generator head to keep it cool. It has 4 separate 20 amp circuits which are plenty of power for 4 vac motors (dual wands), a Cat 290 and PumpTec 207 pumps (higher PSI Cat for tile and grout and/or dual wands).
Get a generator bigger than you think you will need!