In my experience, many over heating issues are cause by a clogged radiator and or intercooler which is in front of the radiator. It's easy to see if this is the issue, lift up the bed and remove the access panel to the motor, you will be able to see a good portion of both the radiator and intercooler which a flash light, and if it's clogged with dirt you will see it! As stated above its easy to address yourself if you want, a good degreaser, I used concentrate simple green, mixed it for heavy grease, used your typical pump up pressure sprayer to coat the rad and intercooler the washed it off with a garden hose. Depending on how hard your drive the MH and the types of roads you drive on will dictate how often it needs to be done. I have also heard some have extended the crank case vent line to extend past the radiator to reduce how fast the grime builds.
If yours is clogged up, normally under light load you won't have an issue but when climbing a long hill in warmer temp's is when this issue will show up! When you pull over the temp comes right back down.