I'm in a position where I will have to do it myself. I've never done it before. I have watched a few videos, but still confused. The gray water tank in my trailer says it's full as well as the freshwater. I went on the outside of the trailer and opened the valve and nothing came out.

My husband has always done this for me, and set up the trailer, so I am starting from scratch on what to do. 

Any tips? Some nights here have been below freezing and I'm worried. 

I winterize my trailer by myself all the time. First, I empty my hot water heater. There should be a rod/prob in there you can remove from the outside. I leave mine out of the winter. Second, switch hot water bypass line so no antifreeze can go through the hot water heater. Third, I empty my holding tank by either opening the lines or running all the water through the sink/shower/toilet. Then I dump my grey and black tanks. At this point my trailer is holding no water and all the lines are open. Lastly, I grab the extra hose attached to my water pump and insert into a gallon of antifreeze. Turn the water pump on and antifreeze starts to come out my sink, shower and toilet. Never had a problem!

https://www.youtube.com/watch?v=ynT0w35oPRc