[Carl n Susan]

It seems to me that to "mount two bikes cross ways above the truck rails in front of the 5th wheel hitch? " means they are across the truck bed just behind the rear window. That works real estate wise until you turn and the RV cap encroaches in that space. Too many here have broken their rear windows with the cap hitting in a turn. This would guarantee damage much sooner.

Or I could be totally wrong about how you want to store the e-bikes.