With all due respect, are you sure it's the mh and not you? How long have you had this mh and how much experience have you had driving it? The reason I ask, is that it is very common for a novice driver or over correct while driving down a straight highway...myself included. From personal experience, I found that was looking down closer to the front of the mh than I should have been. Once I started looking further down the highway, the more comfortable I became with the mh and was able to reduce the amount of over steer.
If you're an experienced mh driver, forget I even said anything. :o