Agreed, muffler has no effect. Heat is not what causes rust, its typically moisture inducing a higher oxygen concentration making it more likely to bond with the iron.

Unless you have an exhaust leak at/around the muffler, and oxygen remaining in the exhaust as water vapor was constantly blowing on the frame.

All rust is fixable IMO. My 98 was way worse than those pics. Over 20 lbs of rust. After spending two days with a grinder and welder and lots of plate steel after removing the gas tank I had it safe and solid again.

The reason people think these rigs rust more than other comparable vehicles is because they last so long.