I also have a similar issue with my 2006 ITASCA Horizon, ISL 400HP. It is on a FCCC. Uses the Sauer Danfoss Hydraulic cooling fan
Usually runs around 179-183 going down the highway at 60 pulling a VW.(any air temp). Thermostat works.
Hot going over Rockies from East on I 90 last Summer, 220 until I got it down into 2nd and ran it at 1700. (93 air temp). Cooled right off after I stopped on top for a while. Back to 179-183 rest of trip.
Hot again last week climbing Stevens Pass eastbound on US 2 in Washington
state Again 220 until I got it into 2nd gear. Stopped on top. This time it stayed between 199 and 208 even when I started down of pass. (98 air temp).
Thermostat works fine. opens around 180.
*Fan always runs*. speed dictated by engine RPM.
Constant dust cloud on dirt. A/C on or off, fan runs the same speed.
I removed the Charge Air Cooler and Radiator today, second time in 3 years.
Both full of dirt and sand.
Unfortunately, Small rocks got in side the fan shroud between fan and radiator and caused considerable damage to the radiator. It is salvageable but just barely.
So, I believe my overheating was due to dirt and sand in radiator. We shall see when I get it put back together and I make a run up to the ski lodge towing the VW.
Last but not least, I think my Danfoss fan controller is not working properly or at all. I am hoping to get some information on the hand made switch built by TWOMED I hope I got that correct.
I only travel very short distances on the dirt, but even that is evidently to much. I would love to be able to turn the fan off completely for short distances, or at least get it to run very slow.
Thank you for any assistance