I would assume it is Automatic Transmission Fluid of some type (Dexron III or Type F?) but I would hope it would state what it is on the cap. Your Freightliner Owner's Manual should have this information, I would think.
Here is a related thread discussing issues with the Power Steering Fluid:
I have a CAT C7 and Allison 6 speed Transmission. On our recent trip, we twice arrived at camp to find our toad covered with transmission fluid? The transmission never ran over it's stated normal operating temperatures (140? to 220?), highest if ever got was 214?. I also read in the manual...