SeilerBird: I guess that if you push my gasser with your DP, I could enjoy a much quieter cabin in my gasser, otherwise, if I just get a DP, my gasser will make exactly the same amount of noise that it makes now. ;D
Seriously, I will never want a diesel: the fuel stinks much worse than stinky gasoline, the exhaust stinks, they make much more noise outside of the cabin, maintenance is more expensive, and they cost much more up front. I will never drive enough to justify the extra initial expense, and will never want a really big MH that only a diesel drivetrain can handle. I would buy a newer gas coach if I wanted to spend lots of money, and pay for premium sound insulation materials and professional installation. I bought a cheap, old gasser because it made sense for my intended purpose, but it has more cabin noise than it should, IMO.
mypursuit: The fan is not always engaged, but it generates some noise that gets through to the cabin anyway. I will consider the option you suggested on the intake.
driftless shifter: I didn't think the sound is Wookie; more like a groaning zombie, maybe? The intake duct is only missing the little flat "cap" that goes over the intake duct at the firewall, but that might make a big difference for all I know. I would expect a duct to work better than the little cap. I have considered the galvanized ducting and the 6" to 4" adaptor, but I haven't tried to piece it together yet. I might also wrap the duct with insulation, like some cars have as original equipment.
Sam!: I have not found a reasonably priced product that appears to have enough sound absorbing/damping ability along with an adequate temperature rating for under the hood/doghouse. I have some one-inch-thick, "sound-absorbing," sticky-backed, foil-on-one-side foam that I bought from McMaster-Carr, but I don't think it will work well for noise suppression in the RV, nor do I expect it to hold up to the heat under the doghouse. It would work for keeping out heat when moving down the road, I think; but once the under-hood heat soaks into it when parked after running, I think the foam will break down. The McMaster catalog has some expensive silicone foam rubber sheet that looks like an excellent choice, but I don't want to spend several hundred dollars insulating the hood and doghouse. BTW, I will not rely on glue to hold insulation up off of the engine under the doghouse; I will fasten it somehow in addition to any glue to avoid contact with blazing hot exhaust manifolds.
I was hoping that someone could point to specific effective material(s) and the correct place(s) to install to get results.
BoomerD: I much prefer earmuffs, and I may use them if a long ride proves too much for my ears.