Seems that tank less hwh is a real bug a boo. I asked Jayco about it and the vibration issue in the 2016 Grey Hawk. I got this response from them.
Girard has made some improvements on there water heater to be able to get an consistent flow of hot water. I have also attached the latest information from Ford on the vibration issue. BCM 1877 – 2016 E-Series E450 Cut-A-Ways and Stripped Chassis Stretched – Vibration Felt And Heard From Floor 55 MPH To 62 MPH Under Load
Some 2016 E-Series E450 Cut-A-Ways and Stripped Chassis stretched vehicles equipped with a 6.8L engine and 6R140 transmission may exhibit a vibration and moan at 55 mph to 62 mph under load. Engineering is currently investigating this condition and is developing a service repair procedure to be available shortly.Do not attempt repair or replacement of components at this time. Monitor QVM and OASIS bulletins for updates.
Some one else solved the problem by putting a regulator between the hose bib and the hose and setting it to 50 to 55 psi.
I don't see the advantage to a tank less heater on a rv truthfully.
I'm trying to decide between a Grey Hawk and a Forester for my next purchase. The tank less heater is on the Con side of the list for Jayco.