This is step by step description and photos for winterizing your camper.  This is brought to us by He Ruide, a very active camper and forum community member.
Your camper maybe a little different but the overall steps should help.

Removing water and adding antifreeze to the toilet
[smg id=550]
Remove the black water tank and confirm it does not have black water from a previous trip. Note the Anti-freeze that is in the black water tank is from flushing the toilet.
[hr]
[smg id=551]
Allow the sight tube to show column (an inch or more) of antifreeze. After this go into the PUP and flush the toilet until 100% pink, undiluted, antifreeze is flowing into the toilet (Not shown)
[hr]
[smg id=552]
Remove the plastic sight tube from the rubber nipple that holds it in place at the top. Completely drain the holding tank. Swing out the fill tube and unscrew the cap to let air in as the water drains out. Reinsert the sight tube into the rubber nipple.
[hr]
[smg id=553]
Unlock and open the cassette toilet door and locate the clear plastic sight tube for your freshwater supply to the toilet (seen on the right)
[hr]
[smg id=554]
Now swing out the fresh water fill neck (turn it upside down if there is residual water). Pour the anti-freeze in until the sight tube shows Anti-freeze.

[Image Can Not Be Found]