I don't know what shop you went to for information, but that is the most ridiculous thing I have heard on this forum. I have towed persons just as heavy as me and this puts the load on an engine as well....and I have never had overheating problems.
I would check your whole entire cooling system. Starting with leaks in hoses, gasket leaks, thermostat, or it could be something as simple as the coolant radiator fan not kicking in. If this is the problem it can possibly be a loose connection in your harness leading to the radiator.
Have a second opinion with another mechanical workshop.