I will take a bit of an opposing view here:
Driver skill, which MOST people do not have when it comes to technical, off road travel, knowing the capabilities and being able to read the trail, etc., is a PART of the equation. Proper vehicle setup is critical for safe travels.
I believe that you should have a vehicle that is MORE capable than what you are going to need. My reasoning is simple:
I can take a 70 year old granny and put her in a Toyota pickup with 100:1 crawl ratio and as long as she can see my hands and do what I (spotter) tell her, she can drive through anything the truck can do. Poor articulation? truck will teeter and tires will spin. Street tread tires? they will spin causing instability and trail damage. You say, " of course that makes sense, but I see people do those things every time I am on the trail. I have seen it from beginners, industry veterans and magazine editors.
Could you get a stock Honda Element with 28" street tires through that same section of trail? Maybe, but I will let you guess which one will be less stressful, easier on the vehicle and MOST importantly, easiest on the trail. The stress level is particularly important if you have a wife or kids who are not comfortable with technical offroading or the family truckster slipping and sliding down the trail.
If you want to take a dirt road with a few chuck holes, to a camp site, then none of this applies. However, the premise of the thread is, "How deep can a 4x4 van get you?" So, I presume the owner (OP), wishes to get quite a ways off the beaten track. Vans are large and heavy, so to get away from it all, you are not going to be driving on smooth, flat, graded dirt rods. A Prius can do that.
For comparison, I will leave you with a few videos. Mind you, they are only comparisons and I am not suggesting that a 4x4 van should be a rock crawler. If you have never driven a well setup off road vehicle, they can be quite impressive, even on "easy" obstacles. Why? the truck isn't working hard at all and a smart driver lets the truck do most of the work. You can be the best driver in the world, but if you have no traction, your truck isn't moving.
First video - "beating on your truck" to make forward progress - irresponsible, stressful and dangerous. Notice how the front end hops over, placing the vehicle in danger of rolling to the side. This is the result of poor driving technique mostly, but another video shows a similar vehicle tackling a very mild obstacle, but due to improper gearing, suspension and tires, the truck bounces up the trail -- not good.
Another bad example. Notice how the 4Runner teeters and spits dirt out from the tires. This is not difficult terrain and could easily be encountered on almost any trail, i.e. wash out, ditch, water bar or even a small oblique rock ledge. As soon as one tire comes off the ground or unloads, you are stuck. With proper gearing, articulation, lockers and tires, this is a cake walk.
https://www.youtube.com/watch?v=uN4aUjsi6wU&ytbChannel=Michael O'grady
Now, contrast that with this diesel Landcruiser in some not so easy terrain. There are no full throttle assaults, no "momentum is your friend", no bouncing to get over rocks because you have 30" street tires and open diffs, etc. Only calm, easy, no damage driving. Can your van or SUV do this for each trail that you want to travel on? If it can't you have the wrong vehicle or are on the wrong trail.
A stock 4WD truck really only has 2WD (open diffs), the suspension travel is not great, nor is it setup to keep the tires on the ground, in an off road environment, which is your goal, no matter the terrain. If you are spinning your tires to get up multiple sections or obstacles, (except for mud), you should not be on that trail or obstacle with that vehicle. When your tires are spinning, you have less control, are more likely to break something and you are damaging the trail.
Here is another example of a fairly capable Iveco EC. Notice the slow speed and proper suspension keeping the tires on the ground (for the most part)
https://www.youtube.com/watch?v=4T6NHiLgJcg&ytbChannel=Martin VanMan
Finally, you should be able to ascend and descend any trail that you plan to travel on without stomping on the gas or locking the brakes. If you are on the brakes going down a hill, you do not have enough gearing in your rig. Descending on gearing, improperly called compression braking, when it is actually the result of engine vacuum, allows one to maintain control of steering and traction
without the use of brakes, which lock tires and can cause lack of steering and speed control.
This is an excellent example of down hill gearing control and the benefit thereof. You do not have to be a rock crawler to take advantage of this. Which is preferred? teetering, sliding down a hill white knuckled with your foot on the brake or a calm controlled descent?
I am quite sure to get blasted over this, but I have driven and/or ridden in MANY off road trucks (Mogs, Pinzgauers, Toyotas, rock buggies, Jeeps on 40" tires, XP1000 RZR, Broncos, etc.) Control is your friend off roading and this is especially true in an unknown area, in a large, heavy expo rig (van) with your wife and kids in tow.
$0.03