You'll need to access the anode rod (WH drain) from outside the trailer. Look around on the outside walls and you should see a door like this:
The fold down clip is at the top; unhinge it so that is sticks straight out and rotate it until it lines up with the matching slot and then pull the door out and down to access the water heater.
The anode rod, and IIRC it is 1 1/16th inch socket, is located here at the bottom:
Take that anode rod out, slowly, and the water will gush out (don't stand right in front of it!). There will be some water left in the bottom of the tank (please remember to shut the bypass valves off before using antifreeze). You can leave that water in there, or better yet, get a water heater cleaning wand and use your garden hose to flush that residual out. It will have a lot of grit and crud that you ought to get out of your tank. Now look at the rod you pull out. It will look awful, but, that's the way it should look. It doesn't need replacing until you can see the steel rod that runs through the middle of the Zinc covering; it will usually wear worse right at the nut...as long as you have lots of Zinc left, the rod is still serviceable even though it looks like it ought to be pitched. I've gotten 5 to 6 years out of our rods and we camp for >4 months per year. Remember to use teflon tape on the threads when you re-install it. The tape will seal the threads and make it easier to remove next time.