If the underside of your Greyhawk is not enclosed, get beneath your MH and see if you can spot where the air is getting into the interior of your coach. If you can, then you may be able to seal it with spray foam insulation. If not, you may have to take it to a dealer and have it repaired.
I am wondering if you are also exposed to exhaust gases from your MH's engine. If so, you could experience carbon monoxide poisoning. Signs of this would be sleepiness and a severe headache. Something worth considering.
Emeritus Member of Jayco Owners' Forum