I would second the choice of a Tekonsha P3. We have had the older version for going on 13 years. It started in the motorhome when we towed my wifes Buick on a dolly. Since then it's been in 2 trucks It currently resides in my 2010 Ford truck and gets used regularly.
The nice thing about the Tekonsha Prodigy and P3 is they're proportional and can be preset for several types of braking scenarios. You can read or download the manual from the Tekonsha site:
They do make adaptor harnesses for most chassis. You can call a Freightliner dealer to see if your particular year and model came wired from the factory. Even if it isn't wired from the factory Tekonsha also makes wiring kits.