There have been several discussions on this. Most operators who have implemented this idea said that any extra revenue generated by trucks was far outweighed by messes to clean up (grease etc).
'd stay away from a truck bay and instead make a 'super bay' for cars/light trucks with more psi, hotter water etc. maybe one of those new undercarriage washers and/or blowers for drying? Just my opinion, though. good luck!