Drive Shaft vibration on 2010 Coachman Freelander

Hello, I am curious what they found out. I have a 2014 minnie winnie 25b on the chevy 3500 chassis purchased new. I noticed a vibration that starts around 63 to 64 mph. I took it to chevy who told me the driveline and frame were extended so winnebago is responsible. It has been at the rv dealership for 5 or 6 weeks now and they cannot seem to fix it....... I would appreciate any info. Thank you.