2000 Chevy Silverado Propane like smell under tow load

Smells problem
2000 Chevy Silverado V8 Two Wheel Drive Automatic 93, ooo miles

I am experiencing a propane like smell coming into the interior of my truck only when I tow a trailer. Typically doesn't start until about 100 miles into a trip. Gets worse climbing hills. The smell does not seem to be coming from the exhaust pipe. All gauges read normal. My truck is my baby and I am worried. Performance does not seem to be affected. I do notice a hard shift from time to time while towing, especially during a slow down at an exit from a freeway. Any help is appreciated. Thanks