Bill, the problem is that there are thousands of self-serve washes out there.
You will have a hard time finding out what their supplier is providing them to clean.
My advice,(and, yes even though I own several detail shops, have to use them sometimes on the road)is to never, never use the "pre-soak" as it is always very caustic.
These washes, depending on the owner/operater, may use products that will clean the hell out of the car, "eats the dirt" off, but in the process, kiss your lovingly applied wax/sealant goodbye!!
If you can obtain some pH strips, check the run off of the cleaning(soap) for it's level.
Most that are the least agressive are going to be around 10 to 10.5. Not good, but then some will have a 4 to 5, on the acidic side.
Seems some think that a hydrofluric or bi-ammonium/ammonium fluride solution is not going to damage a clear.
WRONG!!
As you are aware, any coating you apply to the paint surface is in reality a "sacrifical" coating, it takes the beating rather than the clear.
It can and will be removed by agaitation, high or low pH cleaners. (Dawn, but I still don't understand the use of that)
You are right, get the heavy deposits off, just understand that any cleaner solution that will take off the
grime, will attack and weaken the coating you applied.