If you are only going to drain grey water, you could get by with a standard garden hose and the appropriate hose bibb fitting on the end of your drain pipe run. Install a hose bibb valve on the end of the pipe and you will have good functionality.
Other posters have given links to the standard 3" RV drain fittings and hose options. You can also buy a 3" RV bayonet end cap with 3/4" hose bibb fitting built into the cap. That would allow you the choice of using the standard RV sewer hose or a garden hose. Having that option may be nice if you decide to travel and are away from standard RV dump facilities. Many areas welcome the dumping of grey water near plants and shrubs.
Another alternative to plumbing in new drains would be to collect your drain water in a sewer tote or plastic bucket. That would allow you to dump anywhere without the necessity of doing any plumbing. I don't know how one would rig a shower in the small confines of a cargo van. A better option would be to use an outdoor shower or, if available, a campground shower facility.
