We have the M3 LCD, it came in our motor home & it was defined as having GPS navigation. We are being told that a card can be inserted into a card housing that is wired to the back of the unit accessable only by removing the whole unit from the dash board. Is the M3 LCD capable of providing gps navigation using some type of memory card. If so does the card become outdated? Is there a service fee associated with a card? Or is the M4 LCD the only unit really capable of GPS Navigation?