Here is my 2 cents...
All the extra locks are not going to leave you invulnerable. If someone wants to come in and steal possessions out of your van, they can just bust a window out of one of the front doors and crawl through even if you have a secure secondary lock on the doors.
There are 2 things you need to defend against: 1) unauthorized access to the van and it's internal contents. 2) actual theft of the van itself.
As far as theft of the van itself, the Promaster has a transponder system that is tough to crack, much more so than even the typical transponder systems in vehicles today. A thief would not be able to program a transponder to start the van, even if he had the aftermarket programmers that are available to security professionals now, he would have to have a dealer tool AND access to Internet dealer protocols that are only available to security professionals that have been vetted. If a thief has the technological ability to start your van, then you are dealing with a situation that is indefensible. If he wants it, he will take it. However, that is highly unlikely. Guys like that are stealing Lamborghinis. The second threat in the theft of the van itself, is a tow truck.
As for the first point. When trying to prevent access to the contents of the van, you are only as strong as your most vulnerable point? A thief has no qualms about breaking out your window.
What is your best defense against either vulnerability? IMO, it is a very good alarm system. You can even get one that will trip when someone tries to tow. That is what I will hang my hat on eventually. I wish my van had come with a factory alarm, but it didn't.
All this is immaterial if your van is indeed parked out in the woods somewhere, not within hearing distance of anyone. Even if you have your cargo area secured from the cab, a thief will knock your fancy locks off with a sledge hammer, and come right in. When your high-dollar alarm goes off, he will pop the hood and cut the wire to the horn, only because it is annoying him.
Security is all about minimizing risk. In the world we live in, there is no way to completely eliminate it, we can only minimize it. After all the efforts we might expend to secure our vans, some delinquent might happen along and put a match to it for no better reason than his bitterness, because he his lazy and not willing to work for something nice as we have.
A good alarm and good insurance is my recommendation to minimize risk.