Is it possible to model truck loads as distributed loads in RS2?

Yes, you can define truck loads as either line loads or distributed loads.  Just be careful - all loads are per meter depth in RS2 and Slide.  Below is a comment on tire pressure, which may be useful:

To constrain a distributed load to a certain location, you need to define vertices on the boundaries that represent the extents of the load on the boundary.  See Tutorial 4 for information on how to do this.  Note, be careful with the distributed load.  Remember that RS2 is 2D, so all distributed loads are per meter out of plane.  You’ll want to look at two cases, the case of a distributed load equal to the pressure under the tire, which will be the worst case scenario, and another case where you account for the out of plane spacing of the tires and divide the pressure by the wheel base length (best case).

You could of course also take the entire weight of the truck, divide by the wheel base length and add this as a point load.  If the size of the truck is small in comparison to the slope or structure being modeled, this approximation may be close enough.