I have the definitive solution to your problem, because I have had the same running battle on my 1999 Cutter.
First of all, there was a recall to replace the valve cover breather tube on the engine. This recall MUST be done to prevent a further occurance of your problem. The original breather tube ended at the bottom of the engine. This arrangement allows the engine blowby (which is more than you can imagine as normal) to be collected by the fan, shroud, innercooler, and radiator. The exit of this tube MUST be behind the radiator. Unfortunately, this creates a problem for the towed vehicle unless some sort of catch can is manufactured. (I am working on that)
Next, the radiator and innercooler MUST be removed and completely cleaned. You will not believe the amount of imbedded cruded that accumulates between the innercooler and radiator. Also completely clean the shroud and fan. I recommend that you do not replace the belts at this time. I did that, and in less than a thousand miles, I had to replace the new Green Gates water pump belt with my old belt. (which I kept for emergencies)
If you play your cards right, the whole thing should be covered under the recall, unfortunately, mine was funded and designed by me!
I know this sounds crazy, but I pull a 20,000 lb race car trailer behing my 35 ft Cutter. I have to slow down to 60 and put in in 5th gear on hilly terrain and in 95+ temps to keep operating temps in normal range. I did install a custom made cooler for the transmission with auxiliary fan. This helped before I solved the problem. Now, I hardly have to use the transmission fan.
I put 20,000 to 25,000 miles a year on my coach. I encountered the heating problem in my first six months of use. It took me a year to figure out the problem. I tried cleaning the radiator without taking it out, it does NOT WORK.
I would be glad to help you anyway that I can.