Yours was hardly loaded heavily, they were designed to tow a trailer with a second loaded pallet at a gross weight of over 50 tons. Either it is overheating due to low coolant, a clogged radiator, fan clutch u/s, failed thermostat or you have a faulty gauge or sender unit giving false readings. If you have access to a spare instrument cluster, the gauges are all interchangeable with the exception of the voltage one.