On a Friday evening, because of the tailbacks from the Dartford Crossing, I've often had to queue at Junction 28 to access the M25 for well over two hours. (That's just to join the queue on the motorway!). Because the roundabout at Jn 28 gets clogged up, even traffic which will be heading
away from the Crossing still has to queue for hours to get onto the M25!
The Government and Highways England are fully aware of the need for a new crossing. There has been a massive consultation exercise, ending a couple of months ago. Once everything has been considered, it's hoped that a new road will link the M25 in Essex (between the current Jns 29 and 30) directly to the A2/M2 junction, without the need to use the Queen Elizabeth Bridge:
http://i0.wp.com/www.lower-thames-crossing.co.uk/wp-content/uploads/2015/11/The-route-of-the-proposed-option-1-2.png