[Guy]

We have nearly the same rig as far as drive train and load.We  pull a 26 ft enclosed trailer, and live in our Newell full time. Same high temp issues. The advice given about 35 year old analog temp guages and sending units is so true. The prior owner on our coach had to have a complete rebuild done due to temp problems. He installed a Digi Panel, and it seems to be fairly accurate when I compare to infra red readings on the engine block water outlet. Also gives an audible alarm if you begin to "run on the edge " of high temp, low oil pressure, or high trans temp.The Infra red guns are cheap now, 30 $ and give piece of mind. We had our radiator steam cleaned last summer after a very scary boil over on Baker grade just out of Las Vegas. They did not do a good job, totally missing the corners of the radiator, where you will find the most grease in between the fins. We now use a Radiator Genie hand wand  ( on Amazon), and Dawn soap and lots and lots of rinsing after to clean our radiator every 90 days. So far we have been able to keep our temps down, by doing these things and trying to keep our rpm up in the 1800 to 2000 range on long hills, and some short ones on hot days. 

I believe Clarke Hockwald on this forum has a lot of experience with these issues, as he is around the 60 thousand mile mark of full timing with a classic Detroit 2 cycle, and pulling a trailer. He has been our mentor on this.

Good luck on this, no one need a 25 thousand dollar repair bill . That is what the Detroit Diesel repair receipt says on this coach when we bought it from the prior owner.