Sound deadener for genset compartment?

How much of the noise is transmitted via the air, and how much is vibration through the mount causing the rest of the RV to vibrate? If it's the latter, working out some better isolation mounts would probably be a whole lot more helpful. Increasing the mass of the RV bodywork in the area will help cut down on its vibrations; increasing its stiffness might also help some, as would layering materials of different acoustic impedance (i.e. squishy like foam and stiff like metal) as each change in impedance tends to prevent transmission.

Many foams do not hold up well to gasoline vapors or spray, and quite a few don't do well with heat. The stuff that's there already is possibly as much a heat shield material as it is there for acoustics.