Overheating when pulling my 5th wheel.

So most of the time an issue when pulling a load has to do with air flow .such as clogged radiator fins with dirt that does not let sufficient cooling air reach hot coolant or transmission fluid that needs to be cooled off .or the fan im assuming is a fan clutch .may no longer couple up under load and temp .test this by spinning the fan by hand when the engine is off and cold .does it have resistance or spin freely .usually they should have more resistance when hot an that would verified fan is coupled .also the impeller on the water pump could have corroded fins and last but not least you transmission cooler is sometimes part of the radiator .if it’s restricted the trans fluid will get hot and boil .the excess heat will transfer to the engine coolant via the radiator and cause overheating .a restriction in the A/C liquid line will cause excess heat from the condenser causing over heat problems .im assuming you already have looked for leaks .externally ??