Here's how I have been winterized my 2000 Scamp 16 side dinette with bathroom since the fall of 2006.
Note: this procedure does not require a water heater bypass kit.

1) Fill fresh water tank. (I know - this step doesn't make sense - read on)

2) Take the trailer to your favorite dump station and rinse both the black and grey water tanks.

3) Park the trailer where it will stay for the winter. Place on jack stands if desired. (note: I've not used jack stands since the 2nd winter I owned the trailer, it is over 20 years old now and the axle seems to be just fine)

4) Turn off the water pump, depressurize the water system and empty the remaining fresh water in the fresh water tank.

5) Drain the water heater. Do not install the water heater drain plug at this time.

6) Close fresh water tank drain valve. Pour 2 to 5 gallons (I've used three gallons for several years) of non-toxic RV anti freeze into the fresh water tank

7) Turn on fresh water pump.

8) Close valve on water heater inlet once pink antri-freeze has filled the heater inlet line.

9) Open cold water valve at shower. Run until pink non-diluted anti freeze is coming out the shower head. Run the shower wand in both spray and massage mode to protect all internal parts of the wand. Pump out excess anti-freeze through the drain pump and into the gray tank to protect drain pump and drain lines.

10) Fill toilet fill valve until non-diluted anti freeze shows up in the bowl. Dump it into the black tank to protect the tank. Note: I've been winter camping and had the black water pipe and valve freeze with no damage. If you are concerned about freezing the black tank, dump an extra 8-12 ounces of RV anti freeze into the black tank.

11) Put the shower head in the intermediate (off) position so the shower head is not open. Open cold and hot water at shower. Close cold and then hot water at the shower when anti freeze comes out of water heater tank drain. This step back-feeds the hot water line from the shower to the water heater.

12) Open cold water at kitchen sink until anti freeze appears and close. Open hot, cover the faucet with the palm of your hand and then open the cold water valve at sink. Let run until anti freeze solution appears at water heater drain. Close cold then hot valves at sink faucet then remove palm. This step back-feeds the hot water line from the sink to the water heater.

13) Open water heater inlet valve for a few seconds again to be certain anti freeze is protecting the water heater inlet line. Re-install water heater drain plug at this time. I don't tighten the plug very tight because it will be removed in the spring to flush the water heater.

14) With the water pump still in the run position, press in on the check valve on the city water fitting. Hold in until un-diluted anti-freeze appears. Install cap on city water inlet as normal. 

15) Optional: Drain gray and/or black water tanks again.

Click here to see the procedure used in previous years

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