knocking noise in exhaust

if the cat. is loose in the pipe you should be able to take your hand and pound on it to see if you here the same noise

There is a donut gasket at the bottom of the exhaust manifolds that will come loose and cause the pipe to rattle. It is an easy and cheap fix IF you can get the exhaust bolts loose. All you have to do is get underneath and try to move the exhaust pipe and see how much it flexes at the exhaust manifold.
If it is something loose in either the cats or the mufflers all you have to do is go under the coach and strike the parts. Something loose will rattle.
Do this BEFORE you take it to a mechanic other wise you may be paying for a new part when all that was necessary to stop the noise was tightening up a bracket.

Cats make some weird noises when they come apart. I will also go with the catalytic converter. And fix ASAP. Otherwise it could break up even more and wedge itself in the exhaust and then you’ll have no power and running hot.

Ditto on cat converter. It is starting to break loose internally. It can come loose and restrict the exhaust flow causing burned valves. Don't wait to get it replaced.

Workhorse had some issues with their catalytic converters. It could be a piece of the catalyst banging around.

Richard