Noxious furnace odor

[CaptDaveRet]

I have a Keystone Montana 5th wheel with an Atwood furnace model AFMD35131. It worked wonderfully for a couple of years and then it began producing a horribly noxious odor at the outside exhaust. Initially it would heat fine, but over time not as well and air quality inside would be fine but outside it was intolerable to be near the exhaust (which is located near the front door).
It soon stopped cooling at all. The fan came on as normal, and the heat would kick on but it would not cool and the smell was terrible.
My circuit board is now bad and I am reluctant to replace it without knowing what caused the odor issue. I have been told that the exhaust pipe is not tight inside the furnace so it is sucking in and recycling exhaust fumes.
I am not sure whether to spend money on parts or just buy a new furnace. I have cleaned the pipes; there are no spider webs, bee nests, etc in the pipes.
Any ideas and suggestions greatly appreciated.
T/Dave

[Skip426]

The furnace combustion chamber is not accessible to normal cleaning , just the intake and exhaust tubes.

JMHO : remove the furnace and strip the blower assembly and the burner from the combustion chamber , and flush the chamber with water ; you'll probably be surprised at what you'll get out .
I've done this when helping a friend , the mud daubers got right inside and we got 5/6 lbs of mud out , cured his short cycle issue .

I'm thinking you're issue will be small fried rodent .
It's not a hard job , just time consuming , take pictures as you go and have an old egg carton handy to keep screws organized.
The yellow box in the photo shows the combustion chamber , it zigzags through two layers after the burner ( silver plate ) before the exhaust . This is a Suburban , but your Atwood will be similar. If you don't have a manual they are usually on the manufacturers web site as a free pdf download.