Have installed the dometic 320 in our 2005 and 2010 with no issues using a flex line. In both instances the dometic gasket provided fit perfectly. Dometic has online good schematics with exact measurements. Two nights ago helped install 320 in a 3900 with absolutely no problems.
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Mike and Lorraine
2002 3655 FL, 2005 3650RK
2010 3665RE, 2015 3910FB
F350 crew cab dually 6.7
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