I was fortunate enough to use a friends Hotsey Steam Cleaner from the rear side of my DP. I spent the better part of 40 minutes spraying the cores from side to side. I was confident that the radiator was clean until I read several post which recommended using Simple Green or Super Clean (purple bottle) or Dawn Dish Washing Cleaner. I opted to buy a gallon each of the Super Clean (Wal Mart) , Simple Green (Lowe's). I mixed the gallons of each cleaner plus 2 cups of Palmolive Dish Cleaner in my Garden Sprayer. Started at the rear and drenched the radiator core with the mixture. Went to the underside and sprayed between the fan blades the best I could. Waited ten minutes then began rinsing with cold water. I repeated this procedure at least five times with the engine running and the idle speed set to 1250 rpm. The amount of crud which was released from the inner fan shroud and CAC/Radiator cores was amazing. Having almost a gallon of the cleaning mixture left over I intend on doing the procedure over using hot water as was mentioned by "Granpakoot"
My DP has 57K miles on it and I doubt, from the accumulation of crud, the radiator was ever cleaned before
I've since fabbed a slobber tube extension which extends to the rear of my bus and exits just below the exhaust pipe.
Here's some pics of the Slobber Tube Extension