can you remove the display and open it up and check inside it to see if it has a battery like a watch battery or cpu type battery on it?
or it could possibly have a bad 'holding' capacitor on it letting the charge for memory drain when battery is unplugged.

if none of these suggestions are possible you may need a new display. Not sure an operators manual would help other than to show you how to set it, you'll probably need to talk to the factory on how this display works, a service manual might tell you something but on this particular problem i doubt it. Of course this is all educated guessing on my part and may not apply here but
maybe someone that works for toyota can shed some light on this?