The cost of providing trash service is much less than the revenue I would lose when my customers find another wash to visit that has trash service...
You make a good point of how all car washes are different. My nearest competition is 12 miles away. So I probably don't share the same concern as you.
Regarding: "they just dumped trash even after we pulled out the dumpster." I had anticipated this as a possible problem. I believe it is easy for someone to "add" to a pile of trash. But not as easy to be at a clean, trash free facility and throw their trash on the ground.
A few bags of trash did show up right after we pulled out the dumpster. Always near where the dumpster used to be. We were very diligent to get those removed ASAP. So the local clientele always saw a clean, trash-free environment. In my small town, this worked very well. In the first 3 months after removing the dumpster, I probably got 8-10 bags of trash total. In the most recent 3 months, I have gotten none.
I considered a small trash can at the vac area for lint and small debris from vehicles. But after some discussion, we believed it would be a magnet for bags of trash. So we are staying entirely trash-free.