My 2012 Jayco Jayflight 40FKS. Both the bedroom slides are not sealing up tight when put out. They both leave a large gap in one upper corner. How to solve?

An adjustment need to be made on both of your slides. Depending on the type of slide, the adjustment would need to be done on the lower outside or by inside through the cabinet and under the bed. Making the adjustments are not difficult, getting to the adjustment points may be another story. One important point is to be careful to NOT over extend the slide, this can put excessive pressure on the wall and cause the fiberglass on the exterior to crack. I found out the hard way. Wether the points for adjustment are on the inside or outside, most likely it will be a combination of push bolt with a locking nut. You will need to adjust the lower part of the slide towards the inside to even up the gap. If you can get an actual measurement of the gap, you will want the bottom gap to be just slightly larger; roughly 1-2 mm or 1/16 to 1/8". Once the gap is corrected, now you will have to make an adjustment on the screw drive or worm gear to adjust how far out the slide moves. For an older unit, like my previous 5th wheel, it was easier to remove the anchor bolts from the floor, manually push the slide out to make the slide flush and then re-anchor the drive. However you may end up adjusting, make sure you do everything slow and gradual, keeping everything even.

When I upgraded my bed from a queen to a king, I had to adjust the entire motor assembly to have it centered on the slide for even pressure. While you have your tools out and working on this, take a few extra minutes to evaluate the foam seal gasket around the inside of the slide. If you see any tears or cracks, no matter how small, go ahead and replace it now to save you time and energy later and prevent outside elements from getting in.