Could be a void that wasn't filled. If you can get a light on the other side of the firewall (from the outside). You can then look under the dash and see the light coming through and then seal appropriately. I have had this happen on a couple rvs and you would be amazed how much air can come through a small opening at highway speeds. On the other hand you could have a vent that is not working correctly. What kind of chassis is it?
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2014 Fleetwood Bounder 35k
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