Most trucks have single wheels at the front and double wheels at the back, since the back carries more weight. The front wheel rims point outwards to make room for the brakes and the steering mechanism. The same applies to the inside wheels at the back (but just to accommodate the brakes). The rims are slightly convex to allow the brakes to be positioned as close to the middle of the wheel as possible (mainly for stability).
The second, outside wheels at the back have their rims point inwards. This way, the convex shape of both rims prevents the tires from touching each other under load (the pressure causes the tires to bulge outwards). Another advantage is that the outside wheels can be mounted on the same bolt and the wheel axis is kept short.