While not a direct comment on how it applies to Title II items, it is noteworthy that the SCOTUS views a vehicle (and mobile [not stationary] motor homes) as distinctly different than a home in nearly every critical analysis when it comes to your rights and privileges. It's well established legal opinion that there is a distinct difference as far as the Court is concerned.
For a fair summary and case cites read this:
http://en.wikipedia.org/wiki/Motor_vehicle_exception
Given the Court's distinctions, I would be quite cautious.
I don't have the direct answer to the OPs question. Perhaps it's geared to the person stopping home from the range to get some groceries.... But as a designated, always in your vehicle, "truck/trunk gun..." ... BAD IDEA in general, unless you have some specific tailored reason (I have livestock and I may have to put one down in the field or protect them from predators, for instance).
Slightly off topic, and I started a different thread to address this, but want to chime in here too. After having considered a designated truck long gun, stored away, for many years, I opted against it. Absent some totally unlikely catastrophe that requires the use of a longgun to extract myself, and absent a person being a farmer tending lifestock, or someone deep out in nature or on the plains with no phone reception, no help, perhaps living on the Mexico border, etc. I can see almost zero realistic situations where digging out a long gun from inside the vehicle (trunk, lock box, etc.) is going to be legally justifiable.... or wise. America routinely has natural disasters, hurricanes, floods, earthquakes, riots, etc. The periods of time where folks required and were legally justified in going to get their truck guns are anecdotal at best, and statistically nearly non-existent. Yeah, the Korean shop owners in the LA Riots... the Bell Tower sniper in Texas... okay, there are two and perhaps a few other examples... hardly the norm. And certainly not worth the very real and common risks of theft (rolling the dice every day to be among the millions of stolen vehicles or breakins, for which I have been a victim). Or the risks of unlawfully using or brandishing it and being arrested or prosecuted.
So, perhaps the OP has the wrong question/analysis in mind. What's the POINT of driving around with your $1000 AR, $200 can, and $400 EOTECH.
And - what if it goes south for some reason. Theft, or worse, you illegally use it... A criminal prosecution defense on a felony weapons, assault, aggravated assault, or homicide charge could easily hit $20,000 and quickly exceed 6 digits... conviction would possibly result in a felony (bye bye gun rights), prison (hello bunkmate bubba), loss of job, wages indefinitely, perhaps divorce, etc. and the scarlet letter of being a felon...
From a purely wise or risk/analysis situation, that's a lot of risk and little reward or upside for the security blanket of a tool you'd likely never be able to deploy legally.
Most states require "imminent fear of serious bodily injury or death" for the lawful use of lethal self defense. Not a threat of future harm. It must be immediate. That doesn't mean, time to go back to the truck, open the doors, unlock the gun box, perhaps assemble the gun, insert the magazine, lock and load, and then
return to the "threat"... That is unlikely to satisify the legal requirements for the lawful use of a firearm in self defense.
My advice, stick with a handgun for which you already likely have (or should have) a CPL. Have it available to deploy immediately and defend against imminent deadly threats (car jackings, parking lot assaults, etc.) in compliance with the law, either on your person, or immediately accessible in the vehicle... secure it according to local laws.