Fan clutches work off air temps not engine temperature. There is a bi-metallic coil spring in front of the clutch that expands with air temperature and a viscus fluid is released that cause the fan to engage. Some times dirt and oil builds up on the coil insulating the coil causing it to come on later. Sometimes the fluid will leak out and the fan clutch is no good. I would inspect the front of the fan where the coil is and clean it first. After 22 years it probably has some build up. Also look for fluid leaks. Some fan clutches have slots where the coil spring can be adjusted to come on earlier. When the engine is cold try spinning the fan blade, does it spin freely? Or does it stop after 1 or two revolutions? Another thing not mentioned yet is a leaking exhaust manifold, 460's and 454's are known for that. You will hear a clicking sound with the dog house cover off. Headers solved my overheating problem. Lastly on Class A MH's there is several feet between the grill and the radiator there should be an enclosure from the grill to the bottom of the radiator that forces the air through the radiator and not under it most MH's close off the top part but many don't close off the bottom. You replaced the major parts of your cooling system and said each year it ran hotter over time. Thermostats have wax inside that can leak out over time I would test it as someone already mentioned. Good luck.