dust and dirt in bed under tonneau cover
 

As the title says, the end of the bed is covered in dust and dirt from around the tailgate.
Any suggestions as what to use the stop this

I purchased a tailgate seal, and sealed up all the gaps between the floor and sidewalls (except along the front for drainage) with RTV from underneath. My bed is also Line-X'd.

When I bought my tonneau cover, it came with weatherstripping to put on the bed down the sides by the lights. And along the bottom to seal it up.

I used Rok Block to seal the bottom of the gate to the bed. It keeps dirt and trash and what ever from falling in-between them. I then used simple tail gate seals to get the sides.

Single sided sticky tape foam. I did this first spring on the 2017. Works good, but there is still dust that comes up into the box under the tonneau when doing lots of driving on dusty dirt roads, forestry roads. This is due to the floor drain spots.

I sealed mine with some silicone and also use foam rubber around the tailgate. It still drains what little water gets in with the bedcover closed. Water was less of a concern than the dust that gets in.

On my 19 I took black silicone and sealed between the floor and the sides up front and around my fith wheel plug... cut down on most of the dust.

Install a tailgate seal to create a tight seal between the tailgate and the truck bed. This can help prevent dust and dirt from entering the bed area. Check the weather stripping around the perimeter of the truck bed and replace it if it's worn out or damaged. Proper weather stripping can help create a barrier against dust and dirt. Consider installing a bed liner, such as a spray-on liner or drop-in liner, to protect the bed surface from debris and make it easier to clean. Some tonneau covers come with built-in seals or gaskets to help keep out dust and water. Make sure your tonneau cover is properly sealed and consider adding additional seals if necessary. Routinely clean the truck bed and the area around the tailgate to remove any accumulated dust and dirt. This can help prevent buildup over time. This will solve your problem.