Coming from a long back ground in the trucking business, I can tell you, "your gonna get what you pay for", doesn't apply here! You MIGHT get 3 out of 10 drivers to actually "Wash" their truck! Some care what the truck they drive looks like, others could care less. Then there's the guy who cares but WILL NOT wash his truck because thats not what he's paid to do. Futhermore, who's gonna train all your drivers on how to do it properly (chems etc...)? Now if a guy was getting paid to do the job as folks are here then they might care what kind of job they did. Then theres the aspect of someone purposefully damageing your equipment to get out of work. Happens ALL the time! Automated truck washers are VERY expensive and near all of them only wash the exterior and not under the hood or under-carriage and require someone to operate them.
My best advice is to hire a good fleet washer and take care of him because he'll take better care of you AND your trucks than any driver ever will. I'm not just saying that to get your business because I have no idea where you are, just telling you from experience. I just sold my own trucking company 3 months ago and my Dad has been in the trucking business for 48 years, so I grew up around it as well.