Rear radiators are difficult to get at
, you just have to work at it from both sides inside and out. I use simple green and a spray bottle let it set then hose with a sprayer continue until clean(be carefull about bending the fins). Others spray the radiator down the crank the engine and let the fan help push the cleaner and water through the radiator makes a mess but works. Some folks us dawn dish soap, gunk engine cleaner just make sure they are safe for aluminum. You may also want to look in between the Charge air cooler and radaitor for and leaves paper and so on that may be stuck in there. The big thing is to make sure you rinse with lots of clear water. You can also extend you engines crankcase vent tube to the rear of the coach to help keep from getting the oil mist in the rad.