05 Monaco Cummins 300 running warm to hot

DSDP
That makes so much sense. Thanks for taking the time to help out another newbie MH owner.

On my 05 Diplomat, I would pull the floor up in the bedroom and wash the stacked CAC over radiator with a garden sprayer full of Dawn detergent. I would also use one of those pinching tools from Harbor Freight that you push the button and three pinchers come out to reach debris at the bottom. Monaco's fan shroud is sealed at the bottom. A lot of debris can get down into the base of the shroud and build up. From the bedroom floor, I would carefully reach down with the pinchers and pull out all the debris. Often found a lot of paper and baggies.

Once the debris was gone, I would liberally spray the CAC and radiator from the floor opening. If you can get a garden hose into the bedroom, it's best to spray from there. You can't clean these well enough from the rear engine doors.

Once I did my rinse, I would go to the rear of the coach, open the doors and spray CAC and radiator liberally again. Once done, I would fire up the coach and while laying under one side, spray into the fan blades and allow fan to push everything through the CAC and radiator.

Lastly, Monaco added a funky alarm (not Cummins) that would start to alarm by sounding like a chirping and then get louder to where it finally alarmed. Our brand new Diplomat did this on the way home from the dealer. When the temp started to go up, it would start this very low chirp and increase. We stopped at the base of the Grapevine in California and called Monaco. They told me to disconnect this secondary alarm and forget about it.

I found the alarm under that removable dash panel atop your instrument cluster. Never had another issue.

I know you're on the road and some of the above would be tough, but it is the best way to clean the CAC and radiator.

Finally, if you still have an issue, my next guess would be to look at the thermostat.

Think it is stacked as it is flat all up and down. Nothing sticking out on top of anything else. Just a huge radiator with an upper and lower unit on top of each other.

Tell us how you washed the radiator.

And is a "stacked" cooling system (CAC above radiator) or a "sandwiched" cooling system with the CAC in front of the radiator?

The more details on what you have the more we can help-- Monaco made a number of different DP configurations.

I did a Google for a scanguage-D will look into that for sure....
Been pulling a 5er for 12 years a full-timer and now a DP and this is all new again. WOW
Thanks

Rear

Rear or side radiator?

First of all, you need to get the actual coolant temp from the ECM. I can get my dash gauges to change by turning on the lights. A scanguage-D or something will get this data.

May be counter intuitive, but a higher RPM will aid in cooling, so maybe a downshift under load would be called for. Putting the RPM closer to the max power / torque rating will lower the stress on the engine and circulate the coolant faster removing more heat.

Honestly, I'd be happy with 225° on a 100° day.