Keeping supplies in the vehicle has some ups and downs. Being able to get to it rapidly is a valid concern. There are a few situations where bailing out, ideally with your gear, would be the best option.

But there is also the fact that any gear exposed might make your car a target for theft. Survival supplies should ease your worries. Having to worry that your stuff might wander off while you sleep isn't very comforting.

Also if the stuff is in the passenger compartment some consideration has to be given to what happens to it if you make a panic stop or radical maneuver. Unless the stuff is very firmly strapped down if you slam of the brake it is going to take off to the front of the car. If you happen to be in the way your going to get hit. Having fifty pounds of tools, tins, and bottled water bounced off your skull isn't the ideal way to start an emergency.

My estimation is that there might not be a single strategy that is best in all situation. Stored in the trunk the stuff is secured out of sight and unlikely to bounce off your skull.

If and when flooding seems to be the biggest danger and bailing out a possibility I might move the packs into the passenger compartment but keep them on the floor. Strapped down if possible.

It isn't like Tennessee wasn't at least marginally aware that there had been a lot of rain and flooding might be a problem. That would be the time to move the stuff to a spot easy to get to and to work on driving as safely as possible so the stuff doesn't end up bouncing around the passenger compartment like a pinball.