Do they have a wash pad where the trucks have to washed?? this will require moving them around, or if they just want them washed on their yard,are you able to collect the waste water? all it takes is for some one to report you to the environmental agency to shut you down and fine you.
$30.00 is next to nothing considering you are using chemicals, gasoline, wear and tear, and most of all labor, i agree brushing is ridiculous on the trailer, i may brush the cab and the back doors if they are really nasty. I charge $65.00 and up, for the trucks i wash every two weeks I occasionally use a brush on the cab. you need to be at least the same as or even a little more than a truck stop wash, the truckers themselves would rather pay more for a mobile wash so they don't have to wait at the truck wash. and you should do a better job than the truck wash.
using a downstream injector, The two step process is using an aluminum acid cleaner for the first step, this causes a magnetic reaction with the road film, the second step is with a good strength soap to release the road film. then follow up with the rinse. i dilute my acid, about 2 gallons water to 3 gallons acid, this cuts down the chance of streaking windshields. and all this needs to be done rather quick, don't let the chemicals start to dry. if done really quick polished aluminum can be cleaned without damage using this method. speaking of which, be careful if you are washing polished alum wheels, tanks etc. i typically use a butyl/sodium metasilicate product to clean tires and polished alum wheels. Sodium hydroxide will dull them out pretty quick.
one other thought, the owners may flip if they know you are using acid on their trucks, so i may not mention that to them if you use this process.