I would list it via Facebook Marketplace. It's free and the eyeballs on your ad will be multitudes higher than Craigslist. Craigslist isn't the powerhouse it used to be 10-15 years ago - Facebook Marketplace has taken that crown.
Additionally, I would suggest bringatrailer.com (BaT) and Doug DeMuro's carsandbids.com. Both are auction sites that specialize in oddball, unique vehicles - you can set a reserve auction. I have a friend that had some luck selling via BaT, but I'm not sure what their fee structure looks like. As for carsandbids, the buyer must register their credit card to make an account and they are charged a non-refundable deposit if they win the auction; the buyer also pays a 4% (if I recall) fee, which I believe is the aforementioned deposit.
Your price is competitive, but like others have said, the pool of potential buyers is very small to begin with - not that many people really want a van, even a 4x4 one. Keep in mind that if a buyer could secure a loan, interest rates are high, further cooling the appetite for borrowing money. As mentioned above, the fall/winter is probably not the best time of year to sell. For a lot of people, upcoming travel and expenses are within the next 90 days due to the holidays and it's generally cold outside, so the average person is not daydreaming about camping/hiking outdoors; it's a lot like trying to sell a boat or Jeep during the winter months; the demand isn't as strong as it is in the spring/summer.
I also think the floorplan of your stepdad's van is aimed at a specific buyer. Probably a solo traveler, so that shrinks the buyer pool even more. It's also a '03 V10, the general consensus is the 05+ V10s with the 5r110 5-speed auto transmission are more desirable.
The last big of advice I have is to invest some time and effort in taking some quality high-res photos, in good lighting, in a scenic location. Capture all of the van's features and the blemishes well (this will project honesty). Putting a nice backdrop behind the van (should be easy in northern NM) will do wonders for selling the utility and appeal of a niche 4x4 camper van.
This for sale thread on ExPo is a textbook way to sell a rig like this, IMO:
Best of luck and nice van.