We go into our fifth wheel almost every time we stop. The bathroom, bed, frig, and 2/3 of the countertops are accessible with the slides in, so we find it very useful without putting the slides out. On really hot days it does get warm in there, so we often make a sandwich or a wrap and head for a picnic table or some shade. We don't have a generator or large solar array, so overnighting this way during the summer is not an option for us. We often leave the trailer on the truck for overnight stops in campgrounds, though. I just get it level enough for the frig and slides and stabilize it with the landing gear. Next morning, we just pull in the slides and move on.
Be aware that overnighting is not allowed at all Walmarts depending on city ordinances and/or local management. You can search the web for a document that lists them all and says whether it's allowed or not.
Rob