I often keep my gear including several scuba-tanks in my car, in the Texas sun, during the summer. However, I also have a car-cover, which I essentially always use if I'm going to be anywhere more than 2 hours during the day.
If a scuba-tank gets hot enough for the air to expand about 50% (from 3000psi to 4500psi) the burst-disk in your valve will blow. The result is a steady but rapid release of air, but not an explosion. When scuba-tanks are taken in for hydrostatic testing, the "testing pressure" is somewhere around 5500 psi (for a 3000 psi tank) if I remember correctly, which is well beyond what your burst disk should handle, but less than whatever should cause your tank to explode.
The riskiest thing you could do, is leave Oxygen in a car and that bursts or leaks, because that oxygen turns into a fire-hazard. However, if you use an exterior car-cover, temperatures should stay well within safe limits.
Aside from tanks, UV light and excessive heat could break down (over time) various other materials, like BCD bladders, glues, and o-rings.
Theft is a risk too of course. I'd prefer to leave equipment where it's not visible through a window. In my vehicle, there's a nylon cover which goes over my scuba-equipment when the trunk is closed. My vehicles has windows into the trunk area. I don't know if a car-cover makes your car more attractive and noticeable to thieves or not, but in-theory, a car-cover also helps block any potential-thieves from taking a quick peek to see if there's anything worth stealing. My bigger concern is mostly on a good way to secure my car, when I'm solo-diving, given I don't want to take my car-keys with me underwater, or leave them on the shore. I wrap my FOB in about 10-layers of aluminum, and put it in a cheap $15 safe attached to the underside of my car. Perhaps a motivated thief who knows where to look could get in, but almost nothing is ever truly 100% safe.
In order to quickly use a car-cover, I have two recommendations:
1. Draw arrows on your car cover, on the front, back, sides, and top. Every arrow pointing towards the front of the cover. This allows you to quickly identify the correct orientation.
2. If you're semi-frequently adding and removing a car-cover, you want it to be quick, easy, and secure (including during windy conditions). Sew a few large pieces of hook-and-loop strips ("velcro") in place of the wheel-tie-straps, and be generous with length. Alternatively, add some quality decent 1-inch plastic buckles onto 1-inch webbing (straps).