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3126E Voltage decrease with engine temperature increase

I have a 3126E Caterpillar engine in a 2003 Newmar motorhome application. There is 232,000 miles on the coach, 32,000 miles on the engine, and the alternator was replaced in February of this year.

On a trip across country, the gauges indicated normal operation during the first several hours of driving. Then, during demand for power by depressing the accelerator, the voltage gauge would decrease from about 14 volts to 12.5 to 13 volts, the engine temperature would increase from 180 degrees to 210 degrees, and there would be a slight noticeable decrease in power.

Where do I start troubleshooting these symptoms?