I am "assuming" you are referring to the type pictured below. If not, disregard the rest of this post.
If they are mounted like mine, there are 4 "EL" brackets screwed to the wall. 2 along the top, 1 on each side.
Raise the shade and look for a small Phillips screw along either side Looking toward the rear or front of the trailer. Try somewhere between 2" to 6" up from the bottom.
Once you find and remove one on each side you should be able to pull the bottom out a bit and look up inside the valance. You should be able to find 2 more screws, each about 4-6" from the edge. Remove these and the valance to come right off.
The screws can be difficult to see, buried in the folds of materials.