Will a tummy tuck help with back fat as well?

A tummy tuck will affect the tissue on the abdomen from hip to hip.  It doesn't affect the fat or skin of the back.  That can be addressed by liposuction.  

A tummy tuck will only help the anterior trunk or abdominal area. It will not do anything to improve the back rolls. Options for helping with the back include liposuction or a back lift depending on the amount of skin laxity present.