Hi!
Your're not wrong Zed, just back from Big Sunday (off again in a bit) and the bus is loaded up more, with more in the back, and the volume of the engine must have been reduced by half, though i dont normally have it as full as this, i think the fact the cushions and bedding etc sat flush with the top of the back seat, it muffled the engine really well.
I would have a good at some soundproofing inside the engine bay first, then inside the bed area. Some underlay felt like you used to get under car carpets, maybe with lead type roof flashing to stop drumming type noises, but i reckon a far bit bit of sound is transmitted up and around the side vents area, so maybe a little re-trim in this area too?
I used quite a bit of roof flashing type stuff when i had my interior in bits, it quietened down the clang noise from the side and door panels, but you need some other type of stuff as well, see my gallery below, but as Zed says, your bedding and pillows, duvets in the back does cut a surprising large amount of noise down, i suspect more than any type of after-market stuff.
HTH
Cheers!
Alistair