is this an FE bus? a conventional? im assuming cummins 5.9? whats the outside temperatures and terrain? are you climbing hills? if its warm outside and you asre getting hot on the freeway ,id begin to suspect the fan clutch is not engaging as it should.. this esp true if when its colder outside the bus seems to run cooler.. .. and since it cools right down when idling or driving in town..
if the heat condition occurs when its cold outside even then i'd begin to suspect a partially stuck thermostat, clogged radiator, or somewhat low coolant..
in a rear engine bus the fan clutch is even more importanbt as you cant rely on natural air-flow across the radiator..