Hello and welcome to JustAnswer!
You'll need to do some simple testing with a test light to see if the door lock solenoid on the latch is getting power and ground when the lock or unlock button is pressed. There are a few possible causes for this problem, but the most common would be the door lock solenoid. The solenoid is mounted to the latch at the back of the door and is serviced as a separate piece from the latch assembly. The next likeliest cause would be a break in the wiring harness that runs to the door.
If you have a test light and would like to do some testing let me know. Testing will likely involve removing the door panel, which isn't a hard task either if you've done that sort of thing before.
You have to take the trim panel off to get to the latch.
Start by popping the bottoms of the trim panels loose that are at the front and rear of the door, forward and rearward of the door glass. You will notice that these panels have a clip that goes into the actual door panel. These are just push in fasteners, get behind the pane with a wide flat bladed tool and pry outward or just pull by hand.
Once those panels are free then you can remove the main panel. It's held to the door shell all the way around with push in fasteners. Again either get behind it with a pry tool and pry in the areas where the clips are, or pull the panel by hand in the area where the clips are. Work your way around the whole panel disengaging the clips and the panel will come off.
You're welcome, and good luck with your van! It was great working with you!