Though most are not 4x4, (check out Quigley and Sportsmobile if you want a 4x4 van) there are other benefits to making a van your tow rig. First and foremost is camping. Say it's late fall/ early winter and you want to hit the trails with your trailered 4x4, and the trails happen to be over 3 hours away. Why not hitch the trailer to your diesel van, tow to the trail head Friday night after work and figure you'll get there late after dark. No worries, you had the heater on the whole way there, just crawl in the back with your pooch, significant other, maybe a kiddo or two depending on your family situation and roll out some foam mattresses. In no time at all you're all snoozing with dreams of tomorrow's wheeling.
By dawn the van will undoubtedly be chilly as it's fall or winter outside, but just crawl from your fartsack, crank over that diesel, and get back in your bag for another 20-30 minute snooze. The next time your cell phone alarm goes off you'll be toasty warm in that old plumber's work truck and can more easily get your coffee made and wheeling face on. Much nicer than a frost laden tent that's for sure.
Some folks don't have all the room on their trailer for added toys like motorbikes, bicycles, and guns. Not a problem, haul them along on your extended weekend in the back of the van. Unload just the stuff that will be in the way when you're ready to sleep.
Another benefit of the van over a truck? Conference room! As in, it's cold and windy outside so just invite some of your wheeling buddies into the van, it will be crowded with more than 5 guys, but you can still sit around the cooler and tell lies just like outside, only without all the dust blowing at ya. And the best part, you don't have to walk far when it's time to pass out, just tell your buddies goodnight, kick em out, and flop over on your foam or air mattress.
other tips:
-find a used van with a roof AC unit if you camp in the desert much
-truck bed air mattresses will fit in a van also
-make sure there is at least a back window or peep hole to keep tabs on your 4x4 on the trailer out back should you hear a noise
-add some rear mounted lights that you can easily flip on should you hear someone messing with your 4x4
-van can be used for urban camping also, Here is a great link to a stealth urban camping van write up
-definitely get a mural of Conan painted on the side, it's a requirement. Or at least paint it like the A-Team Van