There's no specific video or manual for the R-21EC as far as I know. However, you can get a pretty good idea of what to do by watching the R-27 Winterizing video on the RT website.
Here's my brief check list for doing the R-21EC on the hard or on trailer.
1) Empty fresh water tank and fill with RV Antifreeze and run faucet until pink stuff flows out then shut off faucet.
2) Pump out the holding tank.
3) Pour RV Antifreeze into Marine head and activate the macerator and wait for pink stuff to come out the hull side port, then shut down the macerator.
4) Close through hull seacock.
5) Open raw water strainer
6) Have another person holding RV Antifreeze jug and have them pour in some antifreeze into strainer with over filling.
7) Tell this person in 6) you are going to start the engine and get ready to pour in antifreeze into strainer.
8) Start engine and with the 2nd person pouring antifreeze into the strainer watch for pink stuff to exit from hull side exhaust port.
9) Shutdown engine and close up the strainer.
10) Either remove batteries or keep them on the charger. If removing batteries trickle charge them once a month until next boating season.
11) Run a small heater such as a Caframo space heater in the cabin and close cabin door.
12) Check the boat at least once per month to ensure all is well.
13) If you have a trailer then get advice on how to winterize it as I'm unsure what needs doing at this time, if anything.
14) Oh.... and fill the fuel tank up to avoid condensation in the tank.
Good luck.
For me, I'm going to place my R-21EC in a covered fresh water slip for the winter and in my PNW area no winterizing will be necessary. I will fill the fuel tank up. I will run a Caframo space heater in the cabin and place a rug over the engine for good measure. Running the boat in fresh water will get rid of any salt water in the engine's cooling system.