The trailer from the 70's would be my last hunt. I'd start at Granny's house, and look for places where people could beat the heat outside, like shade trees. Try to visualize where children played, and where dad's might wrestle with their boys. Such things were done in those days. If there was a shed or small garage, check there, because we lose coins taking a knife out of our pocket, or reaching for a lighter. Find where Granny hung her laundry to dry. Some of the older washing machines didn't spin, and coins sometimes fell from pockets while hanging out to dry. I once read where a man found a small cache of gold coins buried near the base of a clothesline pole at his Granny's homestead. If there is a cellar, you may want to check it out, too, but watch for critters. Spiders and snakes like the cool atmosphere. I have found lots of coins in old yards, but oddly, the more poor the occupants, the more coins I find. The more wealthy the historical owners, the less I would find. I have given thought to this, and think that when people are poor they feel empowered and content with a little extra money. They may give their kids a few coins to jingle around in their pockets to help boost their sense of security. It worked for me when I was a boy, and back then the coins were silver. They sure had a nice ring to them. Boys lose coins. In richer homes, there was not as much insecurity about money, and consequently, parents may not have been giving coins to their children to boost their sense of empowerment and content. Thus, less coins in wealthy people's yards.
There are probably photographs of the mill at your local historical society, or in the library. I'd give them a look and see if there was a parking lot, or an outside area where the employees ate lunch or took breaks. I searched a local cannery once and found two Barber dimes and three Inidan Heads from the late 1800's, along with a 12 karat gold lapel pin set with a small ruby. But the trash was horrible. Use a small coil, and learn to set the detector up properly for high trash hunting.