I agree lot so cars have battery in the trunk, but they are manufacturer engineered with a venting tube or sealed off. A lot of race cars do this as well, but its also recommend to insert venting tubes. Putting holes in your car for venting tubes is risky business - Ive spent ages trying to figure out an easier way to put solar cables and antenna wires into the cabin without drilling in the asbence of pre-existing grommets. I personally would never buy a car where a lead battery is located under the driver seat. Funny you mentioned RV with batteries because I recall watching a video on youtube a few weeks ago about some guy who had overheating problems despite cool ambient temps throughout the RV, and he ended up building in ventilation fans to circulate air in the battery cabinet. RVs have much more air volume than a normal car. I tried to buy a 4-gas detector recently since we transport natural gas sometimes, and it was really difficult to buy a new one. I ended buying an old detector from Peel Electronics, it was the 04-OG-12V which could detect natural gas and battery emissions - companies don't really produce these anymore. Like I said, I call it "over engineering", its unlikely that you will get a bad outcome. But if one does happen, and its cheap to mitigate why not pay for the over engineering? If the fan is really just for the electronic circuit boards, then don't the lithium battery controllers rely on IC as well? Sorry just playing devils advocate.