RV Specific GPS

I have been looking into the RV specific GPS's and have got a lot of varying opinions. The question that I have is what logic does the Garmin 760 and RM use to determine routing for your RV? Many report that when using the RV routing it will rout them to truck routs that are longer or keep them off more direct routing. Does anyone know how this logic is set up in the these GPS's? or is it the same logic used for truck routing?