If the surface feels glass - like after washing, without any hint of roughness, its better to wax, rather than polish. The distinction is that polish contains a mild cutting agent, so you really want to use polish infrequently. It is considered necessary to wax after polishing anyway. Mer is slightly more abrasive than other polishes too, so I would personally avoid it - try it on a dark shiny surface to see this.
When you have washed the van, a quick tip to slow down the build up of dirt, or to use in bad weather when you cant wax, is to rinse with Demon Shine. This causes water to bead off, carrying dirt with it.