Hi myergin: Welcome to Airstream forums. Most of us did not realize you were replumbing your vintage trailer. All we saw was a split fresh water tank and 30 gallons of fresh water spilled in your trailer.
Most campers have the city water on the street side rear. That is where I plumbed my 66 Trade Wind and my son's 69 Globetrotter. We both felt the fresh water tank is a big advantage when traveling. Our trailers make the best rest stop compared to a dirty truck stop. I believe your trailer will be worth less without a fresh water tank.
That said, plumbing a vintage Airstream is straightforward. I personally have never used a fresh water drain low point. I use the compressed air method that seems to work well for me. I don't trust getting the water out using just a low point drain. And I always use the 12v
pump to fill the system (less water heater) with RV antifreeze. Another good reason to maintain your freshwater tank and 12v
pump. But as we say in these Forums, your trailer, your rules!