Since "...everything else is working..." indicates that there is no blown fuse or chances are nor any hardware is at fault.
When the battery was disconnected, the Kenwood KVT-617DVD may have suffered a memory failure and would require a reset.
Have you tried doing a reset? It's a small recessed button near the bottom right side of the font panel underneath the protection panel (right of the Joystick Track/Station Key & above the Eject Disc Key). Pressing the reset button should restore the Kenwood KVT-617DVD to default factory settings. Of course this will also remove any special programming that you may have such as favorite stations, etc.
You also posted "...Parking brake requirement...bypassed..." this is only to allow the display to be always ON. This is the green wire that must be grounded
You need also to check that the "Reverse" wire/trigger is also bypassed that is to set the "...camera...full time...rear view mirror and back-up monitor for the truck camper..." This is the pink wire that must be attached to the ACC of the vehicle's Ignition Switch. Alternately, it may be connected to the red wire of the Kenwood KVT-617DVD.
If the reset does not correct the problem, you may need to run the setup menu again following the user's manual. Should you need a copy, it is available on line here
. Pls verify that you set R-Cam Interruption to ON & R-Cam Mirror to Reverse. This is better explained with appropriate illustrations in page 69 of the User's Manual.
Hope that this be of some help/idea. Pls post back how things turned out or should you need additional information.
Good luck and kind regards.