Not sure that the trim tab will be the issue. It is worth having a look at first, but the normal effect of the trim tab is to neutralise the steering, that is to make the effort required to turn to port and starboard a similar effort.
It would help to know more about your boat. Engine offset is one possible issue, although with a 90 the effect should not be too pronounced, or it could be down to weight distribution in your boat.
I had a very similar problem with my boat. This was down to poor set by the the guy who installed it (me
). I tried to get the static balance right by putting the two batteries on the port side to balance out the driver on the starboard side, as I would mostly be out by myself.
I completely missed the effects of prop torque. I moved the batteries to the starboard side and the boat now handles as it should at speed.