EDIT--Have you new propane since owning the rig? If not could bad fuel be the cause?
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This diagram seems to show a number of vents to choose, but is incomplete on info. Must be a better link for the particular furnace model?
https://pdxrvwholesale.com/products/atwood-hydroflame-furnace-model-afsd20141-tune-up-kitThis is about a different furnace, but might be relevant. Was mention of the gas valve being out of place, and also since air seems to be involved:
"I'm told the inverted louvers are intentionally designed that way to prevent the furnace from sucking in hot 'exhaust' (i.e. CO) air from the chrome 'chimney' below into the furnace's fresh air. This could extinguish the furnace flame."
(Mostly nothing to do with OP's otherwise)
https://www.forestriverforums.com/forums/f113/new-atwood-furnaces-afmd-series-79088.html