If I did not have a low point drain, I would create one. Then with the low point drains open, I'd blow out the lines with low air pressure. Open all the water valves inside to allow any water in the lines to drain. Then I'd briefly turn on water from the source: pump/hose flushing the main lines. I would also drain the fresh water tank, and rinse it well. Then I would add fresh water to the tank and allow that water to flow to each water valve including the toilet and until the water became normal in color and the bubbles cleared. (RV antifreeze in water creates bubbles at the faucet). I can smell the water to see if the rv antifreeze is gone.
For the Black Tank, You can always run a hose down the toilet, if you have no other options. I use a PVC pipe with a 90 on one end and a hose fitting on the other. Be careful not to drop any parts down the toilet.
MADP, Jeep, Blue Ox, SMI AFO, Solar: 900w/672a, Amish cooling unit, Lifetime: SKP & Days End