One of the biggest and most effective anti-theft deterrents to the smash & grab crooks is keep everything out of sight---if they don't see anything chances are they won't try breaking in. Naturally that's not 100% effective but it does NOT attract attention.
Incorporating any deadbolt door lock into an existing PDL system should include a separate circuit that is known as a "5 wire reversal, rest at ground"---here's a schematic:
This DIY or field manufactured circuit is attached to whatever circuit or device controls the PDL's or power door locks. This circuit is separately powered and fused as the deadbolt solenoids draw a bit more current than should be run through an existing PDL circuit. Whether you're adding the 5 wire circuit to the vehicles Remote Keyless Entry (RKE) or an aftermarket alarm having the deadbolts on that circuit makes for a more reliable installation and operation.
The over ride---either my concoction or using The Auto Bolt's pull wire connection fitting---is purely mechanical, doesn't require any electrical power to use it. Even if the solenoid's fail regardless whether power is available or not the over ride works.
Here are a few photos that might better explain what I did:
That shows my solenoid and push-pull cable that's part of that system--the bolt end isn't shown as its not really important to this explanation:
This poor photo shows my over ride--a cable that works to retract the locking bolt when/if its extended. I say its hidden but honestly its mostly just out of first sight, something a crook in a rush might not think to investigate. If they found it works the deadbolt then they'd be useless keeping them out.
That green cable is routed inside the door shell where an opening is provided so it can retract the locking bolt. That looks like this:
That image shows the left/drivers rear door where the typical door trim panel is removed. Since I have DIY plywood panels that would cover the opening shown I have to make allowances for access to that cable. The side barn door has a similar cable that's easily accessible from the passenger's side front door, reaching around the B-Pillar inside a cubby hole I've cut into the door. That too is "hidden" behind some junk I store in the cubby hole, my hope is a crook wouldn't think to look there for anything. Do keep in mind the aftermarket alarm would be screaming when the front doors are opened which is part of my "plan".
The weakness to the deadbolts is the front door windows---even with the deadbolts working those become the glaring vulnerability--to my knowledge there's no way to completely protect them from forcible intrusion. Deadbolt locks can also be installed in front doors but the windows are still very, very vulnerable.
In my case I have a partition just behind the front seats so 99% of my valuable stuff can't be seen or grabbed from the front seats. Most SMB's don't have that separation so the first line of defense would be a loud siren activated by an alarm system. The deadbolts would only serve to thwart the easy lock punch outs but the alarm would still alert a door has been opened.
As for securing your goods while parked at a trailhead I don't know of any way that could be effectively put into practice. Most likely crooks would have the benefit of time to rifle through your van and find whatever you're trying to keep.
I hope this provides a bit of insight into how I dealt with my own deadbolt locks. FWIW The Auto Bolt devices are similar enough to those I used operation-wise so they too could easily be fitted with the mechanical cable-pull defeat. The owner of The Auto Bolt concedes that feature was an added requirement when they began selling to package delivery fleets---the fleet operators didn't want drivers caught inside their own vans.