I do not know what the mechanic is using for a "tester board" never heard of such a thing. However, he should try to scan the car for DTC as a 2003 Mercedes SLK 320 on a R170 platform. That should get you started in the right direction. Was the RCM you used as a replacement brand new? It needs to be new.

Does the car have an automatic transmission? Does the cruise control work?
With what you have giving us to work with you could have a bad battery, bad alternator, bad wheel speed sensor, bad speed sensor on the transmission conductor plate, bad brake switch or brake relay or bad ESP module.

Are there any after market items wired into the car like a push to start, or security item?