OK, I figured out how to adjust the slide so that when closed, the gap between the exterior RV wall and the slide is consistent and it seals properly. On my Jayco, the adjustment is underneath on the trailing edge of the slide. There is not one at the other end -- this one adjustment place adjusts both ends.
See the "can"? When the slide is closed, that bumps the frame. In the pic, my slide is open a few inches. So you loosen the nut at the end of it, then you can turn the can by hand. If as was the case with my TT, the gap was too large at the bottom when closed, you move the can outward so that the bottom of the slide can go in farther when closed. And if you have a gap at the top of your slide when closed, you move it in toward the RV frame to stop the slide sooner.
This was a very simple and quick adjustment, once I found how to do it, so no need to pay an RV dealer. All you need to do it yourself is one large crescent wrench.
I found online that the nut on the very end of the all-thread, is how you adjust how the slide seals against the inside of the RV when extended. That part of mine did not need adjusting, but I understand that is how one adjusts that.
