Ford 460 and first gen V10 are known to breath very poorly. Drop headers an straight thru mufflers on it, and it will make it run cooler and much better responsiveness. Hot engines really don't make good power. It is theoretically possibly that the coach builder did not allow enough cooling in the engine compartment and intake air, which could make the coach sluggish. Get cool air into the intake, and get the hot air out, and it should act much better.
While I'm not some big proponent for Banks, you can see their specs show the power increases available for the 460 and the first gen V10 are substantial. That means there is a lot of available power power that is getting choked off by poor breathing. You don't have to use Banks to get that kind of power, but it is just documentation that shows how poorly the original design breaths.