I had a similar leak on drivers side front slide,for a couple of years. Finally I found the source of the leak. I 1st brought the slide in about 1/2of the way. 2nd I removed the topper. 3rd I cut off (careful not to destroy,it can be replied on) the "D-seal"-rubber gasket that's attached to the top of the outside wall. 4th cut off and remove all the caulking where the top of the slide and the outer wall meet. 5th I went to hardware store a bought a 6" wide roll of "water proof membrane used in home construction to put around new windows and doors. I cut a piece long enough to cover the length of the slide,peeled the paper backing of and carefully folded the membrane from the top of the outside wall and then down and on to the roof of the slide. If you get a wrinkle in the membrane I recommend a small bead of silicon along the edge of the membrane. Put the topper back on and your good to go. Should take about an hour to complete the job. Good luck 🍀 Ron