CHARGE AIR COOLER (CAC) CLEANINGA common cause of overheating in the D (and many other diesel pushers) is a dirty CAC (Charge Air Cooler). The purpose of the CAC is to greatly reduce the temperature of the output air of the turbo. The CAC is basically a small radiator that is mounted in front of the big engine radiator - near the bottom curbside).
One reason the CAC gets dirty is due to the engine oil blowby from the "slobber tube".  If you have not lengthened your slobber tube (or had Freightliner do it) so that it exhausts below the radiator, you will want to do this. Click here for more info on the slobber tube.
Cleaning the CAC is relatively simple. Just get a good cleaner, such as Simple Green, spray it on the CAC, let it sit for about 15 minutes, and then spray with a water hose.  Be careful if you are using a pressure cleaner as the cooling fins are easily damaged by the high pressure water spray.  You will also want to clean the engine radiator while you are at it.
In order to clean the CAC and engine radiator, you will have to gain access from under the bed. However, if you just want to clean your CAC, Leo Everitt has developed an innovative method that does not require access from under the bed. (NOTE - this info is for early-model D's). Here is information submitted by Leo:
I just returned from a 7 month - 8000 mile trip and thinking about having to
clean that D radiator again. The old way never seemed adequate - the need to
get in and around to try and clean it. I decided to look for a different
approach and found that the "bumper" could be removed fairly easily. An
angle bracket on each end is held by two screws and the molding is attached
to the upper panel by 12 addtional screws. Pull the lights from inside (do
not remove lenses) and the assembly lifts right out. The whole job took
about 30 minutes since I used hand screwdriver. Had I got up and went to
tool station and got my power screwdrive I'd guess 15 minutes to do it.