I agree about not re-using the wash water, there will be all kinds of bacteria, possibly heavy metals, all kinds of detergents, silt that will probably pass through filters, etc....
Re-using that wash water will leave a horrible film on the trucks and leave them smelling worse than not washing them.
Find out exactly what they want done, how often, if there will be degreasing, lifting up the box and/or cab, will you be moving them (if you have to move them, do you have the Class B CDL to drive them?), will you have to process or haul away the wash water, etc.....?
Post here what you find out and we will help you as much as we can.