I did want to point out that one of your points is more than a little inaccurate. You cited "automobile exception" as proof your van can be searched at any time. That's not true.
The "automobile exception" gives a lower expectation of privacy, but it does NOT forfeit your 4th amendment rights. So long as there is no probable cause (in this case, nothing in plain sight), you don't have to give consent to search, and any search would likely be found unconstitutional. The safe is, obviously, a great added level of protection.
I just would hate for a van-er to consent to a search without knowing that they don't have to, and get themselves in trouble.
(On the gun piece: I'm not a gun guy by any stretch, but I'm almost sure that, so long as your possession of the gun is legal in your jurisdiction, you could use it in self defense. What you may have been eluding to is 'Castle Doctrine', the right to open fire on someone simply for being on your property. And that's true - you don't get that with a car, so far as I know, in my limited knowledge of that. But if you were in legitimate fear for your life, and your gun was legal in the local jurisdiction, you'd have the right to defend yourself.
Hope to van myself some day!