The collision occurred around 7.05 p.m. on Thursday, July 7, 2022, near junction 5 and in the direction of the Dartford Crossing.
A white Citroen van and a white Yamaha motorcycle were involved.
A man in his forties was taken to a London hospital and is still in serious but stable condition.
Officers from the Roads Policing Unit are looking for anyone who has not yet spoken to police, including anyone who saw either vehicle in the moments before the collision.
Drivers are being asked to check their dashcams for any important footage, and police are also interested in hearing from the drivers of several vehicles that stopped at the scene and assisted before emergency services arrived.
Anyone with information is asked to call Kent Police at 01622 604100 and reference 07-1213.
