I bought some boat engine room sound proofing a few years ago that I wanted to try on a Class C but never got around to it. It was three layers, foam, a rubberized lead and foil. It was sold in 4'x4' sheet if I remember correctly and was very heavy. I bought it at a marine store.
The project never got started but the first problem was figuring out how to adhere the foam side to the walls of the generator compartment because I assumed the foil side should be exposed.
If I were to look into this again I'd check out Hushmat. Advanced RV is using it in their Class B's. Link: http://www.advanced-rv.com/hushmat-n...ction-testing/