Sorry to say But this is normal and trim tabs will not help this. It is mainly the hull design that makes it wonder like that at slow speed, Some boats are worse then others, Tabs just kind of level the hull at speed from left to right and also they lift and lower the bow when needed. There is not much you could do really, But some times you could try to lift and lower the drive a little to find a little better sweet spot some times, But it really does not help much at all. The only thing that may help it out a little better in my opioin would be a Dual prop rotation stern drive.
In the old days before power steering they used to use a anode style tab to help with steering torqe, It was mainly for hi speed steering and countering the prop torqe by off setting the fin a little, And it is still used on outboards to day, But also it did help with some slow steering as well. You would not know till you went out and tested and adjusted it a few times to see if you can find a better spot for both hi and slow speeds. I am not saying it will work but if you had the time to play with it a little you might get lucky with it. Here is a link to them so you know what they would look like.http://www.boatzincs.com/MER46399.html
