Seems odd that there wasn't a by pass installed in your rig. They have been coming standard for quite a few years. Anyway, here is a link to some u tube installs. https://www.google.com/search?client...+pass+in+an+RV
I thought they all had them any more. Have you looked behind the water heater? If there is not one they are not hard to install and come with very clear instructions.
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2009 Seville VF32KS
2019 GMC Sierra 3500HD Crew Cab Denali.
6.6 Diesel standard box.