The dimples are from the spoke heads. It should be easy to see if the rim strip has a problem that lines up with the slit. If you don't see anything that would cause a problem, another possibility is that you are pinching the tube between the rim and tire. When installing a new tube, slightly inflate it with the tire in place and go around the entire circumfrance on both side pushing the tire away from the rim to make sure the tube isn't pinched.
As for longevity of tubes, the tube in my wife's and my MTBs are coming up on 20 years old and are fine.