Turn on your water pump and run the sink faucet until it runs dry, what little is left is not of concern. I doubt very seriously you water pump will laeve 10 gallons of water in the freshwater tank.
For the record my coach a 2019 SOB has NO DRAIN under the freshwater tank. I have fun arguing this fact with others
I actually have 2 sets of low point drain lines, but for either to drain, I
MUST turn on water pump. When I do it not only drains... but drains very fast