Carman, yes, you can get pushed around by semis. That's not uncommon at all in a motorhome this size, including our FR3s. There are several things you can do about it.
1. Insure your tires are at the proper inflation. This can make quite a difference. There is a sticker inside the coach that gives the recommended inflation. Go with that.
2. Make sure your tires are aligned. If you just bought the coach this should be done for free. Ask your dealer to do it as you're not pleased with the handling characteristics. The tires are not aligned by FR (so I've been told), so things can get banged around during the build of the coach.
3. The Cheap Handling Fix (I've done this) can be done by you, if you're mechanically inclined. It involved changing the location where the sway bars attach to their brackets. I do construction, not auto mechanics, so I had a shop that specializes in RVs do this for me for just over $100 bucks. Some people will complain that you're changing the setup that's "designed" by the builder. That's not exactly true. In speaking with a Forest River Rep, he admitted that they don't change the setup from the standard configuration that it comes in from Ford. He HIGHLY recommended doing the Cheap Handling Fix and said he saw not issues with doing so. There are a few strings with this topic, one very long that I've read in it's entirety. Bottom line: nearly everyone claims great gains in handling and I've never heard of a single problem from doing so - plus a FR rep said it would help without any downside.
4. I've heard of people installing airbags and larger sway bars with some success. These things will cost you more than the previous three combined, so I wouldn't start with these "fixes".
Others will likely have suggestions, too. I'd also suggest a trip to a semi scale after you're fully-loaded to make sure you're not overloading one side of the rig. Many people do this to see exactly what the weight is on each tire/set of tires. I haven't done it yet after 18 months, but I plan on doing it this Spring when I get my coach out of storage.
Jay & Jane (Jazzy and Jewel, too)
2014 FR3 30DS
Twin Cities, MN