Yes, sleeping inside your car with your key fob inside will slowly drain the battery of the key fob. But you probably don't need to worry about it. For the full video, click here.
I slept in the back of my 2017 Subaru Outback Limited for 60 nights in a row before receiving a warning that the key fob batteries were low. I got a warning on my dash when starting my car the next morning that the key fob batteries were low. This warning remained for the next 2 weeks until ultimately the battery in the key fob died.
If your key fob is dead:
Do not fret! There is a small siler square on the back of the key fob. Press this reveal a real, physical key that can be used to unlock the doors! To start the vehicle simply hold the dead key fob against the "ENGINE START" button and it will activate the vehicle systems. You can now turn your car on like normal. These processes can be repeated until you replace the battery in your key fob.
How to replace the battery:
First, remove the physical key from the fob. Next, insert a flathead screwdriver into the hole where the physical key is stored. Gently pry off the back of the key fob to reveal the battery. Replace the battery with a new CR 2032 battery that can be found at Walmart for less than $5. Reassemble the fob and you're good to go!
This article is in reference to the 2017 Subaru Outback Limited. The same directions probably apply to the other Subaru models, and potentially other auto manufacturers as well.