Wow!! I have been through this recently. I have a 2006 Holiday Rambler diesel pusher with a 300 HP Cummins engine. Based on my experience I would highly recommend finding a Cummins Service Center. That being said, I've been to other shops for this same problem and at one shop to the tune of more than $3,000 had the radiator removed/cleaned/flushed, flushed engine, new thermostat, hoses and coolant. They also checked my temperature gauge to find it actually reads a bit high. So I added a "Scan Gauge" plugged into the motorhome computer. On my first trip I started receiving check engine high temp warnings where I saw 233 once, upper 220s other times. I finally found a Cummins Service Center where they quickly isolated the problem to one of two items. I was able to eliminate one for them but the other was the thermostat (remember I have a new one already in place). They pulled out the "new" thermostat and quickly realized that it was an aftermarket and not an official Cummins version and the difference between them was considerable. $1250+ later everything works great and I have since driven clear across the US with no further problems. I now only go to Cummins for my Cummins diesel work (including my Cummins generator). This is a long story for me, very frustrating and very expensive so I have condensed it here to avoid going into extensive details. Good luck!
