I’m not sure what this was, but it struck me as peculiar so I took some pictures of it. It’s now being used as a picnic shelter, but appears to have had a previous life. My first thought was salvaged roof from an old barrel roof diner, but I’m not sure. The roof itself looks quite a bit older than the poles its on or the footers, and I can’t imagine the amount of custom fabrication it seems went into this being originally done for a picnic pavilion which could have just as easily been wood and shingles.
So what do you think?
Diner Hunter 
I drove past that again today. It’s not a part of an oil tanker, I don’t think. Maybe you’re right that it was part of a diner. I’ve never been inside a diner with this kind of roof so I can’t really say.
Cross-section is reminiscent of some “transitional” passenger railcar roofs–not the 1937 AAR “standard” cross-section used on the great streamliners, and not the classic Osgood-Bradley “American Flyer” design as used on the New York, New Haven & Hartford, but looks vary similar to an older group of Osgood-Bradley commuter coaches built for the New York Central around the late 1920s that were rebuilt into their first Mercury streamliners in 1936.