the monitor needs 5V so you have to give it 5V which you can get from the batteries.
the easiest way would be (if the batteries deliver more than 5V) that you connect a linear regulator like the L7805 http://uk.mouser.com/ProductDe.....fc%3d
, or more of them depending on the current the monitor needs, to the battery and the power lines of the monitor.
since the monitor is going to be a usb type you have to connet the regulator to the + and - lines of the usb cable.
there are existing cabels to slave in power to a usb device.
you could then add a transistor or rather a mosfet between the regulator and the batteries which will allow you to "swich" the regulator/s on and of.
those transistors of fet´s can be triggert by an gpio from the raspi. a gpio is a input/output pin on the raspi board which can be set to high or low (on/off) by some kind of software.
hope that helps..