We've overnighted at WalMart, Cracker Barrel, Love's and TA truck stops. We've NEVER disconnected and most WalMart's that allow overnighting don't allow you to disconnect. We try to park at the lot edge with the slides overhanging the grass (watch for sprinklers, they can destroy the underside of a slide and flood the trailer).
In August, your biggest issue will be air conditioning for a comfortable overnight sleep. I've never seen a WalMart that allows generator use with a "stand alone generator". Running one in a motorhome is completely different and you'll hear them if you walk past the "camping area" at most WalMart's.
The process is to park (usually where you see other RV's) and go inside to the Customer Service desk. Ask about overnight parking. They will either tell you the "local rules and where to park" or they'll call a manager who will give you the information.
Keep in mind that there is a growing number of locations where overnight parking is not permitted. So, be sure to inquire before going to sleep. I'd suspect it's no fun to be sound asleep and at 2AM, get a knock on the door from the "just coming on shift security guard" informing you that you've got to leave or be towed......
There's a Yahoo Groups site called "walmartrving". On that site, there's a downloadable PFD listing of all WalMart's that allow overnighting and a partial listing of those that do not allow overnighting. There's also a fairly complete listing of the rules and expectations as well as a "group compiled set of rules on WalMart overnight self imposed guidelines"....
You can read the "WalMart Corporate Policy" here:
https://corporate.walmart.com/freque...walmart-stores
"Can I park my RV at a Walmart store?
While we do not offer electrical service or accommodations typically necessary for RV customers, Walmart values RV travelers and considers them among our best customers. Consequently, we do permit RV parking on our store parking lots as we are able. Permission to park is extended by individual store managers, based on availability of parking space and local laws. Please contact management in each store to ensure accommodations before parking your RV."