Most RV fires alongside the road are caused by mechanical or electrical problems, usually MHs. It's rare to see a TT or 5th wheel burning on the road.
For TT or 5th wheels it would be brakes as the primary cause of on road fires. But, I've only read about a few of them.
From what I've read a majority of RV'ers travel with the refrigerators on propane so you would think there would be a lot more refrigerator caused fires on the road. It seems the systems are fairly safe. Most of the RV refrigerator fires I've seen photos of happened while the RV was parked in a campground.