I have a 2009 Silverado 3500 HD duramax. When the truck was new I pulled our 14,000 pound fifth wheel through Eisenhower Tunnel (11,000+ feet). The engine and transmission both overheated on the way up. The high temperature lights did not come on. No problem since with overheating. Most towing is on flat land with one trip west each year. Is this normal Any modifications to avoid nonoccurence?