Are there tents that attach to vehicles?

The Right line Gear SUV Tent lets you sleep off the ground in the comfort of your own vehicle. The tent connects to the back of any size SUV, minivan, wagon, or pick-up truck with cap.

How can I stay cool while sleeping in my car?

3:0912:00LIVING IN MY CAR: Stay Cool in Your Car During Summer - YouTubeYouTube

Can you stay overnight at a Walmart?

Yes, you can park overnight at Walmart. Most Walmart stores allow RVs, van-dwellers, and car campers to sleep in their parking lot overnight.