Temperature gauge not working

I do not have a scan tool I could use. The temperature gauge is on a 2000 Damon motor-home on a freight-liner chassis with a 5.9L Cummins turbo diesel engine. The temperature sensor has been replaced. Also we checked the gauge by disconnecting the 2 wire cable at the temp sensor and took a paper clip and bridged across the 2 terminals of the wiring harness. The temperature gauge went from 0* to 280*. So it was determined the gauge was good. I have tried 2 different sensors that so far will not register on the gauge. I also took an infrared gun and took the radiator temp. Went from 65* F to 140* after 10 minutes of engine run at 1,800 RPM's. This has me baffled after 51 years of auto and diesel repair. I guess the next step will be to replace the temp gauge. Wiring checks okay, no shorts or breaks. The coach is a diesel pusher with the engine 30 ft from the gauge. To scan the engine it takes a special connector that is not like the OBD connector. It is one that Cummins Engine people have. It cost $175.00 to scan the engine. Any suggestions?