Wondering if I start the phantom-establish home point- take off -i get in the car and drive 5 miles down the road and establish new dynamic home point and press RTH buttom-turn off controller will the Phantom come back to the new home point 5iles down the road?

I have a feeling the battery would not last that long

Yeah,  battery life may be right on the edge there,  but if you did it,  say 3 miles away it would work,  so long as you had good gps coverage.  You would also have to either have someone at the initial launch point to send it off,  or have a clear line of sight to send it off because of radio limitations.  I have flown my Phantom over 3 milkes before,  but that was with enough altitude to have line of sight for the radio.  If you launch the Phantom,  then jump in your car and outrun the Phantom  that might work.  
But the easier solution is to just tell the Phantom to land at the last waypoint instead of creating a new dynamic homepoint. I am not sure if you can do this on the Dji-Go app,  but you can with the Litchi app.  One of the nice things about the waypoints is that you do not need a constant connection with the r/c.   But you do need to make sure all your altitudes and travel lines are clear of obstructions.  I think you might also need to go to the final waypoint and establish for sure the exact ground altitude above the initial home point.  A fun experiment to try,  but maybe costly.  I am thinking,  even if you set a new homepoint dynamically,  that point is best set a bit above ground level.  Because the Phantom uses the barometric altimeter from the initial home point to as the reference point for altitude.  Maybe another user has tried this and has a better insight.

The battery life would be okay.  I have flown more than 5 miles on one battery many times.  The problem would be the Phantom recognizing the new home point and the RTH signal.  You would be out of range so how would it know any of that info?

I have been planning a multi-flight way point mission for months now.  What I do know is this:  If you set your flight parameters carefully, know your flight distances for the batteries you are going to use, plan very carefully, you CAN set the P3P on a waypoint mission beyond return capabilities if you are also there at the critical point to take control of the quad.  But this also requires that you set the RTH limits and battery limits as far as distance go to minimum.  Otherwise the quad wants to return to the original homepoint and, at least for what I am trying to do, is impossible.  It will not have enough reserve.  But I can say that yes, you can set your quad on a flight to a point beyond the return point and regain control and land it.  I have never let it land on its own.  But I have flown it to beyond the safe return to home limit on a 'suicide' run to see if this was possible.  It is, but if traffic, any obstacle prevents you from getting to the spot needed to regain control, the quad will be toast as it will just run out of power.

Kneepuck Posted at 2016-2-15 06:08
Yeah,  battery life may be right on the edge there,  but if you did it,  say 3 miles away it would w ...

yes, it will get back to the new home point, however i also would like to have somebody on the initial take off point
your statement about the RTH hight is very important and maybe even crusual in this case

Also, don't forget to factor in the time it takes to get in the car and drive 5 miles down the road, this is effectively lost time and is using up the battery.