2003 6.0 No Power Towing

[nebrigich]

I have a 2003 6.0 that runs just fine with everyday driving. However, if I even tow 3,000 lbs I end up around 30 mph at the top of a long hill. I was told and read on some threads that it could be the oil cooler. I will get that checked soon, but anything else I should ask about when I take it in? Is this something that a code will indicate if I have the codes checked? There is no smoke or anything. I had the head gaskets replaced in January because it ran very rough when it was cold and the head gaskets were leaking. Any help would be GREATLY appreciated.