I came across some USA data on Carbon Monoxide poisoning inside houses where storm power outages caused owners to use back up generators, and placed them within 7 feet of the house.
Not just in the US, unfortunately. The only fatality during Cyclone Yasi was from a fellow using a generator incorrectly, and suffocating. I think people underestimate just how little carbon monoxide you need.
Back on topic - xcavator - whilst I think your idea is possible, I suspect it would be far too much trouble than it's worth. That Rob Gray had a problem with his gen is concerning - he's been around the block enough times to know which is the sharp end of the stick. That said, big boats run gennies in their engine rooms in a very similar setup to what you're describing (external intake, external exhaust) and don't have any problems with it, so I'm sure it's possible. I would suspect that by the time you built a sufficient muffler for the gen though, you wouldn't have much in the way of rear cargo area left. I wouldn't like to refuel the gen while it's in the car either - I have a habit of spilling petrol everywhere even when the genny is sittong on the ground at my feet.
I know it's not the cheapest option, but if it were me I think I'd just have to save up and plonk in one of these:
http://caravansplus.com.au/catalog/product_info.php?products_id=10881(ebay has them slightly cheaper, plus I've seen them on getawaeco.com.au every now and then)
Get it installed by a professional gasfitter, plumbed into the gas tanks on your camper. Pull up, open camper, turn on gas, instant heat. No fumes, no generator running, no petrol needed, no problems. Granted, it's pretty much a single-use piece of equipment, unlike a genny that you can use for a lot of other tasks.
That's my $0.02. If you do decide to go ahead with your original plan though, I'd be very interested to see how you go with it. It's certainly possible, and I've learnt to never underestimate the inguinity of swaggers when it comes to figuring things out.
Good luck!
Matto