Carol,
Do you use Gimp or Libre Office? (both free, and available for Mac or Windows)
Gimp is a photoshop-like program. but you can also paint with it, and draw lines and arrows and so on.
Libre Office is a office-like program which has a drawing tool. Again, you can draw lines and rectangles and so on. It's not like visio with any sort of predefined electrical images or the like, but you can certainly use it for basic drawings.
For that matter, It's fairly easy to flip Sketchup into 2D mode, and then just draw outlines, or objects that you can move around. The neat thing about sketchup is that you could first draw it out by hand on paper, and then scan the paper, import that images into sketchup as a model, and then use that as a backdrop/guide as you then draw the actual model in sketchup. For that matter, you could model the trailer in 3D also, if you wanted to use it to try and fit in your tools and benches and so on.
I also had a quick peek at the Grizzly online shop planner, but it does not seem to have anything in there for electrical hookups and the like.