You don't say what model hot water heater you have, but they are all similar. There is a video on how to do a Suburban here:
https://youtu.be/3GJYTNpCvyo
Below your trailer there should be three drain valves. One is usually by itself and that is for your fresh water tank. The other two should be close together and one is probably blue and one red. They are your low point drain lines for draining the lines that run to the sinks, showers and toilet. Open all the faucets to break the vacuum and drain the lines more completely.