2006 GMC Savana 3500 6.0 engine overheating pulling travel trailer

Hello, the first thing I would do is verify that you are not exceeding the gross vehicle weight of the vehicle - the gross vehicle combined weight rating, as well as ratings for tongue weight. Check all the ratings and ensure you're underneath the limits. All components of the cooling system should be checked - ensure your thermostat is the proper temperature rating, ensure the radiator / reservoir cap is rated to the proper pressure and tests good. Check the heater core for proper flow. Ensure the water pump is pumping coolant appropriately. Verify there are no air pockets in the system. Ensure there is no debris restricting free air flow at the condesner, radiator and coolers. Beyond all that, if everything else checks apparently good, it would be quite unusual but you could have some kind of internal engine coolant breach as well. My guess is something has been missed or is out of specification. Good luck. Seek guidance from a local shop near you if needed: https://repairpal.com/repair-shops/gmc-repair-and-service-in-el-centro-california