Hi burati1... New owner myself, just went through process... I'm betting your unit has Antifreeze inlet on outside of trailer. First thing you need to do is make sure your Hot Water heater is off. Uplug it from 110V power source, make sure propane switch is off. Release water pressure valve (mine is accessible from outside panel) Be careful as water may be hot! Empty hot water heater by removing plug from bottom of panel, mine is 1 1/16" nut. Once water heater is empty, replace plug. (it will have anode if Suburban, willnot if Atwood) Next you need to find hot water heater bypass valves. Make sure water flow bypasses water heater. Open all water fixtures in the trailer and find the low point drains, open them. Ensure that all fresh water leaves your plumbing. Remember to flush toilet until no more water cycles through. If your grey and black water tanks contain any waste, you will need to dump them before doing anything. If they only contain water they can be flushed with sliding valves. My trailer was brand new, did it in my driveway. So... with all water purged, you can think about drawing Antifreeze into your system. (make sure you are using non-toxic RV antifreeze!!!) Close all your taps. Connect a short hose to Antifreeze inlet. Put other end of hose into Antifreeze jug. Turn on RV water pump and Antifreeze should be drawn into your system. (my trailer took just over 2 gallons, but is smaller than yours) Once pump is drawing AF into your plumbing, open taps one at a time until pink stuff (AF) flows freely through the line. Make sure enough AF gets into the traps. Flush your toilet until enough AF will protect black-water tank. Make sure shower head gets flushed(including outside shower if you have one) City water inlet (next to AF inlet if like my trailer) can be purged with AF by removing small screen and pressing the backcheck valve until AF flows out undiluted.
Experienced People.... Did I miss anything?
Hope this helps... Norm
2016 Silverado 5.3L Z71 with tow package
2012 Rockwood Mini Lite 1905
Distribution Hitch: Trunnion