gee, I'll have to keep an eye on that one...wouldn't you know, the one week I don't check e-bay, and a manual for a '73 appears...Oh, well..it'll probably go for too much $$$ anyway..
Dan: I also have a 73 safari 23'. (mine came w/ a manual...but no service manual). You want to open the valves under the sink that you mentioned, but also, there are several under the bathroom sink. The manual says that you should let all the water drain out, then jack the trailer all the way up, letting anything left in the pipes to drain to the back, and out the drains. When the water stops, do the opposite, lowering the jack all the way. Some more water will find its way out the forward drains.
I don't believe that the manual says that you need to pump antifreeze throughout the water supply lines; they do suggest (as many people here do) that you get a "blow-out plug" and attach it to the city water inlet, and use compressed air to blow out the lines. Then they say to pour a cup of antifreeze into the traps, (sinks and shower),into the black tank, and toilet.