...I used Seam Grip (crystal clear) on the **Weblon** soft walls (I don't know what your softwalls are made of, so do your due diligence before applying) on our pop-up truck camper window stitching (the window bottom stitches). That was almost 17 years ago. The Seam Grip is still in perfect shape after all this time (we still own the 2005 camper). I never had to re-seal in all these years.
But be aware: you REALLY need to investigate where the stitching leaks are. I found our water beads (not terrible leaks) during a post-tropical storm in the Outer Banks, when the rain was coming down sideways in 54 mile per hour driving storm.
Just be sure that the water infiltration isn't coming in along the vertical stitches somewhere!
Caution: Seam Grip volatiles smell like he**. And, it is flammable until it dries. I applied it on our inside stitching (seems to work perfectly in our situation). You may want to consider applying to the exterior stitching. If you use this, you had better practice on stitching not associated with your camper (some scrap stitching on some vinyl). You cannot remove this stuff if you mess up the application technique. Also, the cured material will darken slightly after 4 to 5 years (unless they changed the formulation since 17 years ago).
Good luck. This worked for us. Your mileage may vary.
Seam Grip:
here-->
