I was unable to find the vent located in the bathroom vanity. I cut a small hole in the vanity cabinet and it appears that it is directly tied into the vent that goes thru the roof, KEYWORD appears.
I looked under the kitchen sink and found a cheater vent. Ran over to Lowe's and purchased a new one. The rubber flap in the new one seemed to move back and forth a lot more than the one I removed. Also the one I removed was barely screwed into the piping.
I also looked at the washing machine but with no access panel in the closet I would need to remove the washing machine to investigate further.
So... installed the vent, ran water down the kitchen & bath sinks and fingers crossed the problem has been solved.
2013 Berkshire 390BH / 360hp