This Yahoo group is the best source for Walmart overnight parking:
The list owner publishes a spreadsheet of the overnight parking status every few months. It's literally where many sites get their info.
The other option is to just call the store and ask "if they allow overnight RV parking?".
As for Kansas, the only thing I know is that I visited Monument Rocks National Landmark off of I-70 in very western Kansas. Awesome place and the kids enjoyed it. Not historical- just some rocks.