I use one gallon.
Here is my procedure:
Open all drains including the water heater and drain full system.
Close all the drains and replace low drain plug
Hook up my compressor to the fill valve and open and turn air on @ 20 # pressure. I open faucets and blow out lines. I do move from faucet to faucet throughout the coach after initially having them all open. When completely blown out I disconnect the compressor.
I open all faucets and remove the heads on the kitchen faucet, Shower Head, toilet sprayer head, disconnect water and ice lines on the fridge as well as the main water line feeding the fridge and remove the fridge filter, I leave the water heater tank plug loose. I disconnect the lines to the water pump which on my rig is very accessible.
Last step I put antifreeze in the sink traps and some in the toilet bowl.
I do not put any Antifreeze in my tanks as I do not want the residual taste
Make sure all black and gray water tanks are empty