Chasing down old exhaust leaks is quite simple. One person plugs the exhaust pipe while another is under the rig listening...the pressure builds high enough hearing aids are not needed. 454 engines cook exhaust manifolds, I would start at the engine, and work towards the rear.
Now if your exhaust manifolds are leaking,salvaging them is highly doubtful. I've always used a scorched earth approach, heat the studs up to a cherry red with a high heat torch..acetellyn...shock the bolt with a quick burst from a water hose and then begin to break the bolts loose. I've always rocked them...loosen till you hear a pop, then righten then...back and fourth until they finally loosen. Never power them off,more than probably you will snap the head off right at the neck. One can also use a large hammer and beat the manifolds, that too will shock the rust..but RV s generally do not have the room.
Sound brutal ? It is...total war take no prisoner's. Personally I detest exhaust work, a tourch and a air chisel are the essential weapons of war. Lift the RV take off the wheels,dismount the heat shields. The only finesse here is heating the exhaust bolts and breaking them loose. Heat the bolts quickly and beak them loose slowy.
The above is merely a opinion..did I say I detest exhaust systems..I've never seen one I've liked.
