My M915A1 runs cool also. I usually don't see more than 150 degrees with the AC on running bob tail. It did get up to about 160 degrees on a long grade pulling my M373 trailer. I've checked the top radiator tank with an IR thermometer and it's 10~20 degrees hotter than the dash gauge, still not up to 180 degrees though. The dash gauges all read a little low on my truck. If I remember the electrical diagram correctly, they are all grounded at the same points. I have checked and not found any bad connections. I just include the error when reading the gauges.
As mentioned above, I think you are going to have to load the truck to get the temperature up.
