If by 'switch' you mean valve, then yes you will put it to winterizing mode and suck the antifreeze from the bottle. Saves you from having to put it in the fresh water tank for the pump to be able to push the antifreeze through the system. You don't want to put antifreeze in the fresh tank... it takes forever to flush out in the spring.
As to how many gallons you'll need.... depends on how much plumbing you have (every rig is somewhat different) and whether you blow out the lines first with air. It will take less when blowing out with air because most all of the water will be evacuated from the lines and so when the pink stuff starts coming out, it will not be diluted.
Usually 2-4 gallons is sufficient.
2016 Flagstaff 8529IKBS Diamond Package 5th Wheel
Ford F-Series SCREW 4x4
TST Tire Monitor - Sinemate 3500w Gen.
2016 Nights Camped = 46