Well, I'd say the first thing you want to do is determine if you are building a big teardrop, or a standy travel trailer. If I woke up in your shoes, I'd look through the Hall of Fame and see what sorts of designs grab your attention. I'm building a customized version of Dale Summer's Squidget.
http://www.tnttt.com/gallery/image.php?image_id=85372 Get a feel for how much room you want, and what sort of features you want. Like for example, to sink or not to sink? RV toilet with a holding tank in a real bathroom, portapotty, or use campground facilities? If you decide to do a full-on RV bathroom, look at StandupGuy's build.
http://www.tnttt.com/viewtopic.php?t=43655 Once you define exactly what you are building, chances are, somebody has already built something similar, and their build journel will let you learn from their mistakes and cut down on your own
Best of luck!