You can buy apu's now that are inverter generators and are super quiet. There are several different brands that make them. Not all APU's have generators that create AC power. Some are just a/c compressors with a DC alternator. Some also rely upon the vehicles cooling and ac system to operate, so do your homework before purchase. Look up "go green apu" if you want an example of an alternative manufacturer.
As far as buying one from a wrecked truck, I did just that.
I bought a pair of apu's from a salvage auction for 100 bucks. They were in rough shape so I had to engineer everything else involved. Fuel system, DC power system, AC power system, Engine protection systems, Cooling system, Fans, etc. They were basically nothing more then a yanmar diesel engine with a sincro generator head, and even the generator head shorted out so I upgraded that too.
Is mine whisper quiet? No, it's about the same as a standard gas generator. I have more sound deadening to add if wanted, but inside, it can't be heard over the noise of the ac fan, so I'm not worried about it.
It runs off the fuel tank of the bus, so no gas cans to deal with. We ran it close to 50 hours over the weekend a month ago and consumed around 25 gallons of fuel.
Has a 40 amp 12 volt alternator. So the bus batteries are always charged. You don't have to worry about using interior lights or cell chargers or anything.
Has a 6kva 120/240 volt generator, which powers my rooftop AC, sound system, and lights just fine.
I'm happy with what I have, but if I were to do it again, I'd likely just buy an onan diesel gen set and be done with it.