no no no, you need to stop the patch on a patch on a patch solutions and go right after the source. Jack that cap up and put a better proper thicker seal all around the outermost area of the bed lip where the cap makes contact. One person with a couple 2x4 chunks can do the job. Unbolt it, lift it from inside, on your back, insert the 2x4 pieces vertically to support the shell on one side while you set the new seal, then do the other side / front. measure your headroom, make the 2x4s ~3-4" taller.
Then the seal you linked earlier will take care of the rest.