Navybanker is referring to Cahokia Mounds and visitor center that is east of St. Louis.
I cannot imagine any stretch of river road nicer than Rt. 100 between Alton, IL and Grafton, IL. It is better viewed traveling south as you are in the lane closest to the river. It is a 2 lane divided highway between the base of the beautiful bluffs and the river. I believe it is about 16 miles. Take a little tour of Elsah, IL in your truck. Google Grafton, IL and look at "images". Grafton is at the confluence of the IL and Mississippi rivers. Just to the north of Grafton, IL is Pere Marquette State Park.
I would get on Rt 96 east of Hannibal, MO then Rt 100 by Hardin (bridge over IL river) then south to Pere Marquette SP. There is a life size chess table on the floor in the lodge. The campground has electric. No FHU. There is a nice drive up above the lodge with several lookouts and you can go back down a different way back to Rt. 100 that is also scenic. You end up about 3 miles south of the lodge. Grafton, IL is a neat river town with local restaurants, wineries. The Loading Dock has a nice flea market on the 4th Sat/Sun April-Oct. The Aeries Winery view is spectacular!
You can take your truck/toad across the Grafton Ferry($) in to St. Charles County, MO and visit old St. Charles (river history, brick street, old buildings w/shops, brewery, restaurants, casino). The Brussels Ferry is free across the IL river in to lower Calhoun County, IL. This is nice hilly scenery. You can also take the Golden Eagle Ferry ($) over to St. Charles County, MO. I don't think that I would trust GPS on the MO side as it is just country roads between the river and St. Charles.
If you are just passing through take Rt 100 to Alton, IL turn right at the light on Rt. 143 to Route 3 to I-270 to I-255 and you will cross the Mississippi just north of Columbia, IL. See the confluence tower and Lewis & Clark Museum near Hartford, IL.
Happy travels,
Marcia
Dave and Marcia
2019 Ford F350 Dually
2013 Lifestyle LS34SB