You said you took the heater out of the camper and can still smell the burned smell, correct? That rules out a lot and isolates it to JUST the heater.
Doing the sniff test; is the smell coming out the vent that goes into the camper to heat it?
How about sniffing the exhaust vent? Perhaps the exhaust fumes are getting into the chamber/exchanger and mixing, then going into the camper.
I would recommend buying a CO sniffer at the local hardware store; the type of sniffer that has a digital read-out. Put the sniffer at the exhaust and see what that reads. Then put the sniffer at the duct and see. If the smell is unburned hydrocarbons, you'll know it for sure this way if exhaust gas is getting into the duct.
My bet; you have an internal baffle that wasn't installed right in the exchanger and exhaust odors are getting into the heater output duct. If it were me, having the heater already removed, I would not put it back in but I would call Atwood again and arrange to mail or UPS it to them and insist they replace it. You may have to bite the bullet on the shipping, but my bet is that they would cover the cost.
I've dealt with Atwood directly on a camper jack. They mailed me a replacement AND a return shipping label on just my say-so of the problem. Good folks if you talk to a technical person or engineer. At least for me.