Whether driving 6 or 15 hours a day, there is always a Walmart off an exit to rest up at. We do that all the time when putting on the miles. We park overnight with other RVs far from Walmart's active areas.
Walmart welcomes over-night RV parking with few exceptions. For example, Galena IL is a tourist town with an in-town Walmart. No overnight parking is allowed, this to provide business to the RV parks in town and the motels & hotels too. If they did allow it, maybe there would be more Walmart RVs then shoppers.
Big city Walmarts often restrict parking too, this to prevent homeless people from setting up camp. There is a website somewhere that lists overnight Walmart parking.
As a return courtesy, we will always buy our daily needs there before heading out.