The manuals really don't help with much--too generic.
Google that specific trailer model and see what pops up. There may even be an owners group somewhere. Surely there is some info out there.
First basic rule is to get the battery(s) charged and hooked back up--everything else depends on this. Get the them back in and see what comes on when you flip switches.
Make sure you label the wires when you disconnect them from the batteries.
Back then, some of the light fixtures may be 110 volt, so you will also need to have it plugged into power--chances are, the trailer has a 30amp plug and you will need a 30amp to 15amp adapter, hopefully there is one with the trailer already, but easy to come by at a hardware/rv store.
Have you come across this website? http://www.tincantourists.com/wiki/doku.php?id=mallard
Also might be something here:
Should be some info there.