I also have a 2012 390BH and had the same drifting problem. I did extensive investigation as to what was defective and also what made it defective.
The drifting can be caused by either a defective hydraulic manifold valve, the hydraulic cylinder or the ant-creep valve mounted to the cylinder. I had the cylinder and anti creep valve replaced; about a $1,100 job. Fortunately I had insurance to help.
I have found that because the dinette (sofa) cylinder is the longest,
it can have unwanted bending forces applied to it because it snakes over a chassis beam and under the timing rod. Although the cylinder is meant to extend and retract, the movement of the slide-room makes it go up and down, which tends to have a prying action applied to i
Refer to the following two threads for more...........
Slide-Out Drifting Troubleshooting
Hydraulic issues with slide and jacks