I also have the rope lights under the trailer. I have dark purple and it works well for lighting up the steps. The rope lights are attached to the I-beam frame using plastic 1/2" Kwik Clips. The clips have an adhesive back and stick really well to the I-beam. Clean the beam surface with rubbing alcohol before attaching the clips.
I put them on in the Spring of 2012 and leave them attached when traveling. I have only had one clip come off, and it separated from the adhesive back which was still attached to the I-beam. I just applied some super glue and all is well. There is a 1" hole in the I-beam right between the tires and that's where I tuck the plug and excess cord of the rope lights into when traveling.
Here's a link to the Kwik Clips that I used: Kwik Clip, Plastic, 1/2" diameter at Menards