Road Mate and Interstate/Rest Area Destinations

I recently purchased a Road Mate 5366T. I am attempting to save some custom/frequently used routes into the unit. When attempting to manually add an interstate as a destination the unit will navigate to a local road near the selected point. I tried a few things to get it to work; I even went as far as typing in the co-oordinates that I got from Google maps, I got the same results. Then as a test I attempted to navigate to a rest area on a nearby highway by selecting it from the stored POI’s. I again was given directions to a road adjacent to the rest area. What gives? Any advice on adding an interstate as a destination?