If there are only two places to connect wires to the switch itself, I think you have the wrong switch. This is a link to the switch you should have: 3 Way Ceiling Light Switch 510195  - $26.95 : Out-of-Doors Mart!, More Airstream Parts on-line than anyone!
In my 78 Excella it was wired like this:
The black wire coming into the fixture from the trailer is (+), which supplies power for all of the lights. If your light also has a fan, this same wire supplies the power to the fan. This black wire was connected to the black wire on the switch. If your switch's wires are labeled, connect the black to the wire labeled "power".
Each bulb holder is riveted to the fixture, which provides ground (-) for each bulb, so there is no ground wire to connect.
Look at the black wires that come from each bulb. You should see that there are two circuits, one high and one low. On my trailer two bulbs were connected for low light. The second circuit had four bulbs connected together for high light. The switch is made so that when it is in the low position only one wire is energized. When in the high light position, both load wired are energized.
The other two wires leaving the switch might be labeled "load". One wire leaving (the one in the center position between the other two wires) the switch should go to either two or three (low) bulbs. I do not remember which color. The other wire leaving the switch should go to either three or four bulbs (high). If your trailer is wired 3 bulbs on each circuit, the load wire positions does not matter. On my trailer, when the switch rotated from the off position one click, two (low) lights bulbs came on first. One more click rotating and all six bulbs burned. One more click on the switch returned to the off position.
Hope this helps!