A couple months ago I drove up to Milwaukee for a concert and I noticed this phemonenon at least a dozen times on the way once I got north of Tennessee:
By the interstate there would be a pond between the interstate and a farm, I can’t tell if it is a retention pond or if it was built there for the purposes of the farm, but it’s retention-pond size. There will be lights next to it and definitely places to park and a road leading up to it. Occasionally there will be an RV there and/or a bench to sit by the pond.
Is it an actual RV park/campground? Is it just a place where the farmer will go out occasionally when he wants to hang out by the pond? Or a little from both columns?
The one I see like this in Ohio just south of Findley on !-75 is definitely an RV park. More than just a farmer and some of his friends hanging out. Seems weird to want to camp right next to the Interstate but meh, whatever. Imagine it’s pretty cheap. Right down the road from the truck stop and the Adult bookstore; so the clientele must not be too discerning.
A single parking/trailer spot? Most that I know of are there as a fishing camp. The farmer already has the retaining pond, so you stock it with fish. Then you take a break, bring some beer and spend a few quiet hours sitting on the bench and fishing. The trailer keeps your fishing gear, some snacks, and maybe something to pee in (you are next to the highway). If you have electricity there, you can also keep the beer cold and the fish pond aerated.
I can’t say for sure this is what you saw, but a lot of inexplicable roadside ponds are old springs or artesian wells that have fallen into disrepair. In the days before cars and even in the early days of motoring, having a water source every so often along the road was important and so when they were laying out the early roads, if there was the opportunity to go by a convenient spring they’d take it. They’d sometimes even make some sort of improvements to the spring and build little camping/picnic shelters. The springs aren’t so important in the days of powered wells and Interstates, but the Interstates were usually laid down right on top of or next to the old routes, so they still go by them.
The Midwest is also pretty big. When traveling thru it I like to stop someplace close to the Interstate for convenience. Not close enuf to hear the truck traffic but close enuf so that I can find it again in the morning!
Some of these RV stops are very nice. Some aren’t.
Like Qadgop said, in the flat Midwest these are often borrow pits. The earth was piled up to make the nearby overpasses or to fill in a swampy area. Some enterprising person thinks “pond I can stock with fish” next to “Interstate for visibility and traffic flow” and starts a campground.
The less said about Greasy Jack’s dubious theory the better.