Find a spot at the edge of the lot, go inside and ask permission from the manager. If he or she says yes, then by all means extend the bare minimum of slides you need to get around in. Walmart does extend we RVers this courtesy, so by all means show your appreciation by going inside and getting something like a deli rotisserie chicken or pizza for dinner and some donuts for the morning. And don't worry about those people who are taking you to task for asking about Wallydocking. It's only a rest stop, not a destination.
The local Walmart here has its lot full of eighteen wheelers and RVs every night. It's right off the interstate and shares its shopping center with the usual suspects in fast food, a great Chinese buffet, a sit down Mexican joint, and a Cracker Barrel. It's a great place to grab a meal, restock supplies, and get some rest. The lot is patrolled by the town police and when the DW and I moved to Tennessee, we spent our first night there. We slept like babes in our mother's arms.