There is a huge debate on this subject, but here is what I know from personal experience: it’s perfectly fine to store water in your vehicle for long periods of time. It’s perfectly fine to store it in your hot garage or shed.
There’s nothing wrong with letting your water get warm. Warm water is nothing to be worried about.
Many people worry that when the plastic heats up, that it’ll seep toxins into the water. However, simply letting the bottle get warm isn’t going to cause any harm to the water.
In saying that, you want to keep it out of direct sunlight. The direct sunlight COULD cause damage, but it’s also not a guarantee. It depends on how long the direct sunlight was hitting it, if the plastic even got hot enough, etc.
Store your water under your seats and/or under blankets or other such items to keep it out of direct sunlight.
Rotate the water every 3-6 months, just like you would rotate your other water supply.
Water is perfectly consumable if it’s warm.