Final troubleshooting for Ford 460 motorhome, advise?

Yeah they can do that, but thankfully due to the age of this vehicle there isn't even a downstream oxygen sensor after the cat or on the cat itself. Just a single sensor prior to the cat.
Exhaust Manifolds --> Header Pipes --> Inline 02 Sensor --> Converter --> Muffler --> Tailpipe.
EGR system feeds from header pipe as well as the top of the cat.
Sweet and simple. Haha