I have a 2006 5.4 with 130,000mi. I recently got a camper and pulled it a few times with no smells or issues (actually, the truck performed very well). On my last trip to a campground close by (only about 20 mi away) I smelled a burning rubber smell when driving. The smell was not present when idling or when truck was off, but returned when driving. I immediately stopped and inspected everything. There was no smell under the hood, by the tailpipe, by the truck tires, or by the camper tires. The smell was not detectable anywhere. I disregarded the smell and thought it was just some road rubber on the tailpipe or some other external outdoor smell. We camped two nights and hooked up the camper to drive home and smelled it again when driving. When I got home, again, nothing could be found anywhere and no smell was present. I just changed spark plugs and COPs about a month ago. I pulled my trans dipstick and the fluid isn't gritty with no smell or burnt color. I'm baffled as the truck performs well and no signs of issues, but the smell is horrid. Any ideas would be greatly appreciated. Thanks.