If the slip indicator is one ball out, rudder trim may be a factor, but adjustment of the wing cams may also be required. The angle of incidence of each wing is adjusted until the plane flies well coordinated hands and feet off. When doing this, it is nice to have the two wings adjusted so they are either both "forward" or both "aft". One of each will result in an aircraft which remains a little more difficult to trim.
Sustained flight from full tanks with one ball out is certainly going to affect your fuel flow balance, and your butt too, I would think!