RE:Would this work with Hydro Vac trucks?Yes and there is a but.
PTO runs a pump that runs the hydraulics that runs the equipment and it doesn't easily turn. They have been playing with hydrogen for this type of application even plug in on site. (crane booms and even vac trucks) but the many batteries you would need would make the vehicle to heavy and not able to run the roads.
Stright trucks have limits and even a heavy duty truck can only be just over 80,000lbs and these trucks with loads are that much.