Turning Radius -- Can it be calculated/estimated?

I took my fifth wheel to a local trailer hitch service/repair shop for an annual inspection. I was planning on backing out, but the technician told me 18 wheelers turn around in their lot after inspection, which surprised me. I was able to turn around. I went home and checked the distance using the distance measure tool in Google Earth, then noted it. You might try the same thing in something like a school or mall parking lot, even using traffic cones and a long tape measure if necessary.