So on your final slide step, your left foot comes back toward the center of your body, correct?  That's actually not a bad thing.  It helps place your balance toward the center of your body.  Many pros do it It's possible something else is causing your balance issues, such as early or late timing, poor body position at the line, etc.  Without seeing you bowl, we can't really be sure (especially since those problems almost always are just the result of something earlier in your approach).

It is on my final slide step that my left foot ends up pretty much mid-lane. By what both you guys mentioned I seem to think it is probably my posture/balance because sometimes by taking that step left I somewhat fall away from my slide to the right and my right foot ends up near the gutter. I like the idea of marking where my slide foot should end up, thats a good idea.

you  also  could  be  trying  to  target too  far  to  the  right  for  where  your  slide  is.   this  in  turn  will  also  pull  you  out  and  make  you  fall  off  the  shot.

if  your  slide  foot  is  on  20  then  I  would  try  hitting past  9  at  the  arrows.

you  could  try  and  use  the  dots  (either  foul  line or  the  next  ones  out)  in  line  with  the  arrows  to  keep  your  arm  swing  in.  learning  this  will  stop  the  reaching  toss.   a  closer  target  is  an  easier shot  to  duplicat and  allow  for  greater  flexibility in  your  game.