If I remember correctly the Six Pacs sinks were plumbed to the outside where you would connect a garden hose and fill up a bucket or blueboy tank. There was no holding tank.
For the sink there was either a manual pump type faucet or in yours probably an electric pump all in one faucet....these were used on boats and adapted to RV use. There would be a fresh water jug or tank below the sink area that could be filled from the outside on some or removed and filled on others. A very simple design and effective plus low cost to manufacture.
A faucet similar to this
http://www.westmarine.com/buy/jabsco--galley-pump-faucet-kit--P011_331_001_501
