OVERHEATING 2003 Gulf Stream Diesel

[Gene]

I have a 2003 Gulf Stream Friendship With Cat3126e Diesel. I have always experienced Overheating on very long upgrades. Have flushed & cleaned the system. Replaced water pump belt, thermostats, & cleaned exterior of radiator. While on a long upgrade,& while it is running hot I can cool it down by turning on the heater. That works until that additional coolant becomes hot. .

On my last visit to the repair shop for an unrelated issue I asked them to check the water pump belt again. They said they tightened it & charged me About $60. for doing so but I'm not convinced. The belt is extremely difficult to access as it is behind the harmonic balancer. I have driven approximately 2200 mi. since they supposedly checked it. When I just now personally inspected the belt it appears to have a small amount of deflection, maybe 1/2"-3/4". The sides of the belt also seam to be glazed, like maybe it was slipping.

It is hard for me to believe that after all of this time & 75K miles that my issue could be just a slipping belt. Could belt be that sensitive?

I really would like to here from some one with a similar problem. Today I am beginning my return trip from AZ to PA & have waited too long to act on this issue.
Gene Malone