first find the leak,
fill up the tire to max psi,
spray/pour soapy liquid on the tire, can use window cleaner, dish soap mixed with water, anything that will bubble up when air blows through it,
spray the bead, and stem,
then start spraying the tire itself, all over the treads
best to jack up the tire or remove it, so you can rotate it as you spray,
look closely as sometimes bubbles can be small,
when you locate the leak, if its on the tread,and not huge can plug it yourself with cheap plug kits,
i have plugged many holes and they have never failed me after plugging it correctly,
on vacation once i even used bar soap mixed with water from the hotel room to make a soapy mix to find the leak,

 

What Terasec said but try to immerse the wheel assembly. It may not be the tyre/tire and you can save a lot of time. Wheels can leak too. A large washtub or bucket works great. Even if you can only immerse half at a time it is better than spraying soap.

 

For really small pin hole leaks, I find windex to work best. You have to spray the windex on very slowly though to minimize bubbles from just spraying it on. Soak one whole side of the tire down then wait. After 10 minutes or so you'll find a small, but very foamy area somewhere. flip and repeat if nothing shows up.

 

Buy a can of Fix a Flat. Wait untill the air is low in the tire spray the can in and fill the tire with air. I've done it a few times, never fails.

 

In Canada, Wal Mart sells green slime.
Same as Fix a Flat above.

Once you put that stuff in you have to drive for a bit to disperse it around the tire.
Plus many tire shops have a special fee added on if there is crap inside the tire.
I believe its called the "this azzhole put slime in his tire" fee.

You don't mention what kind of wheels on the van...don't overlook a leak from it. The stamped stock steelies on a lot of older units are going away now, after winters of salt and such. If you take the wheel off and look at the inside diameter of the hub hole you may just notice rust and separation. Those can be fixed by wire brush, dripping some solder into the seam and painting over it...then selling the thing soonest.

If its a continuous outer wheel with the centre hub welded in check those welds...same fix as above. Sometimes if its a wheel leak you have to remove the tire to find it.