I would not put the antifreeze in the water tank, it will be a pain to get out. Here is a link to Winterizing
. But basically you drain everything, put a bypass on the water heater, and pump the piping system full of RV antifreeze and pour a little down the drains to protect the traps. IIRC on a AS of your vintage you have to disconect the pump too as part of the winterizing process. Hopefully someone with a similar vintage to yours will be able to give more specfic directions. As far as the location of the control panel....
On my 1975 it is over the galley, on my buddies 68 it is over the fridge in it's own little cabinet.