While you are getting those critical engine cooling items inspected, as RV Wizard wisely advised... Have a close look in the immediate area between the exhaust system and the point where the air line(s) were heated enough to fail FOUR times. GOSH!!
Were all 4 failures at the EXACT same point on the air line? If so, your search should focus almost exclusively on a direct line-of-sight from that point on the air line to the nearest exhaust component. Obviously, if this is repeatedly happening and it has never happened before; something has CHANGED.
Look for signs of a missing HEAT SHIELD which may have been preventing "radiant" heat emmited by the exhaust system from cooking the air line.
Definitions: 1. "Radiant Heat"- travels through the air like the rays of the sun and warms objects which are not shielded from its light-wave spectrum; ranging from Infra-red through the full rainbow of Visible Light, to Ultra violet. Light IS heat.
2. "Conductive Heat"- travels through physical matter and transfers heat to anything in physical contact... a motor which is running will get hot to the touch as the heat from the internal combustion process is "conducted" through the steel... and, yes, it will eventually become warm enough to begin producing some detectible radiant heat.
Sometimes, when exhaust or engine work is performed, shields and shrouds can be inadvertently left out in the re-assembly. (Not an accusation aimed at mechanics... just a real-world potential cause/effect scenario.) Yes, it happens.
Look for mounting points, brackets, or fender-bolt receiver holes where fasteners may have at one time been attached to hold a shroud in place. Look at any old pictures of the motor area you may have ...to see if you can identify a component which is no longer present.
Finally, when you have eliminated any engine cooling issues and everything is totally up to specification... meaning that your engine is operating well-within safe temperatures... IF the problem still manifests itself;
1. Consider wrapping the exposed and affected air lines with high temperature foil-backed radiant barrier insulation available in a roll at a hardware or auto parts store. This will effectively stop up to 90% of the "radiant" heat. ...And I seriously doubt that "conductive heat" generated by the combustion process, which spreads relatively slowly through solid parts could EVER cause such a failure to your air lines before the engine block turned into molten steel and spilled out of the engine bay to the road below.
2. Have a qualified mechanic fit the area around the suspected offending exhaust components with a metal heat shroud which can be easily fabricated from galvanized sheet stock. Care and attention must be taken to avoid restricting any airflow which the automotive engineers designed to allow cooling for the motor ...and thus potentially creating yet another problem. A good mechanic will know how to do this correctly.
Good luck in your pusuit of a solution. Let me know what you find out.
All the best to you,
Jim and Fran in Central Florida
1989 Country Coach Sedona Mark V
Turbocharged Detroit Diesel 8.2L "Fuel Pincher"