I have a 2015 17B, always leave home with the fresh water tank full, empty the gray and black tanks only when they're close to full, my maximum speed is 60 mph and have not had any vibration issues.
Before the last trip the fresh water tank was full, the gray and black tanks each about 1/2 full and I had about 30 lbs of propane. I don't store anything heavy in front and the tongue weight was just over 350 lbs. Do you know what your tongue weight is? I'm not an engineer by any means but wonder if perhaps by loading the heavy stuff up front you're masking the vibration problem.
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