Extension Cord

I'd still like to hear why the trailer cord needs to be unwound. There are thousands and thousands of 30 amp motor homes out there that have the power cord plugged into an outlet powered by the generator. The cord is usually wound in a very small space (mine is about a foot square), and I highly doubt that anybody unwinds the cord when running off of the generator. Air conditioners are routinely run for hours at a time via these coiled cords. I've never heard of a problem or incident with this setup. I always thought that 30 amp trailer cords and 30 amp motorhome cords are the same. So what's the problem?