"Lake" in this part of the country usually means a flood control reservoir on one of the river systems. Many of those are operated by the US Army Corps of Engineers, which builds recreational access areas, but few resorts. Some early Corps lakes got private resort development (Lake of the Ozarks, Grand Lake O' the Cherokees for example) but Congress banned such development from new Corps properties in the latter part of the 20th century, so we got campgrounds, wildlife preserves, and wildlife management areas instead.
500 miles from St Louis will get you to the Cumberland and Tennessee River systems winding through Kentucky and Tennessee, with the more important resort type properties at the north end of Land Between the Lakes. These rivers drop down into Alabama, but you will find more parks on the lakes in eastern Tennessee.
500 miles will get you to the Corps (and private) properties on the Illinois and Arkansas river systems, most in eastern Oklahoma, southwest Missouri, and northwest Arkansas. Because of the timing, you will find more resort properties in the Arkansas corner, around Rogers and Bentonville. The Branson area is more popular as vacation destination, but most of the waterfront RV opportunities are public campgrounds, while the resorts in the area are not necessarily on the lakes, rather more convenient to the attractions.
Wisconsin, look along the Mississippi and Wisconsin rivers for COE parks and state parks. These will be more often riverfront rather than lake properties. I like Wyalusing SP near where the Wisconsin meets the Mississippi, and Devils Lake SP north of the Wisconsin Dells, a resort area built around a reservoir in a gorge on the Wisconsin River. I'm not sure the Dells are within 500 from St Louis, but it is close.
I also like Mark Twain Lake, only 120 miles from St Louis, but it is not so much a resort area as Lake of the Ozarks, Branson, or Land Between the Lakes, if that's what you are looking for.
