When ours did it in 2006, the 2 solenoids for that slide had to be replaced. It sounds like it could be your "in" solenoid that's not holding, although our slide came out a whole lot more than a couple of inches, while on I-84, so there may be another cause. Does the slide work properly when you set up and break camp? Pumping fluid through by activating the slide may wash out problem debris from the solenoid seals.
The manifold of solenoids is likely under the right front of your year and model of coach. The O rings associated with a solenoid can fail to seal due to a debris particle or other factor, and they can theoretically be attended to. But HWH recommends replacing entirely both, at the same time, the in and the out solenoids associated with any one slipping slideout. They can offer instruction and diagrams on their website.
I wish I could tell you which set of solenoids in the manifold go to which slideout, other than that any one set is opposite one another, so you may have to have help with that. You might want to call HWH, or Beaver Coach Service in Bend, for advice and perhaps a repair facility reference. Bretz RV in Missoula comes to mind.
In the meantime I'd get a piece of 2x2 or 2x4 to place inside behind the drivers seat, between the coach wall and slide lip while on the road, to keep it fully in.
Joel