John,
I had a similar problem when I first plugged in to a 15 amp outlet in the garage at my old house, using a home and garden 16 gauge power cord. Since we have moved, the coach is stored about 2-1/2 miles away, and the owner of the lot lets me plug in there. Based on Bill Sprague's advice, I bought a 100 foot 12 gauge extension cord, and although its a long way, the 12 gauge apparently is the difference. No problems since.
Stan