Gomirage;
First, as a former superintendent of a large wastewater treatment plant, I would issue you a permit based upon the following:
(1) Max daily flowrate (gallons per day)
(2) Oil & Grease (remember you are washing food trucks) I would require installation of a grease trap.
(3) BOD
(4) COD
(5) TSS
(6) pH
(7) TPH (total petroleum hydrocarbons)
(8) Chlorides and TDS
(9) Dissolved Oxygen
(10) Heavy metals
(11) Ammonia
Likewise, I would require some of the above parameters to be monitored online/realtime and installation of a SCADA system in order to monitor via a computer at the WWTP.
Many municipalities have begun to request car wash, truck wash racks, etc. to recycle their wastewater. In the long run you will be pushed in this direction. Likewise, by achieving zero discharge now, you are forgoing in future discharge limitations.