It simply takes time to get comfortable in MHs. They are wider than taller than we are used to. Thousand miles was about when I relaxed...........
Good advice to drive the speed you are comfortable with - a little apprehension is a god tickler to adjust how you are driving.The best advice I was ever given is to "drive your lane" - eyes need to be down the road a bit. Focusing too close in front of the RV can cause lots of oversteering - weird but looking farther down the lane seems to mellow this out.Try to ignore the trucks and focus on your lane placement and all will be well.
In heavy winds, slow down. Making sure you have proper tire inflation. In a short time you will learn to trust the feel of the RV as well as your driving. Happy trails!!