Brand: Thor Motor Coach
Model: Palazzo 33.3 34'bunkhouse
John, I've owned my Palazzo 33.3, a very similar length and model floorplan, for over 100,000 miles across this country, Canada, and even Alaska, back in 2017, and have had none of the issues you describe, other than a slight 'pull' to the right during the first year, which was easily solved by Freightliner Custom OASIS Service Center, in Gaffney, SC, where the chassis is built - they simply swapped the front two steer tires - no problems since.
I will admit that you are driving with extremely high PSI tires - you should simply weigh your coach, FULL LOADED, which is with people, 3/4tank of fuel or more, 1/2 tank of water or more, food, clothes, stuff, etc., and THEN refer to your tire owner's manual to determine the CORRECT PSI levels for your front and rear tires - you'll probably find that your fronts will be in the 85-90 range, and your rear duallies in the 90-95 range - for a MUCH SMOOTHER RIDE.
I have NONE of the additional steering and suspension additions that you speak of, they are not needed. These chassis' are built for a motorhome, for this application, and the XCS chassis height is a perfect combination with the wheelbase(around 17' I believe), and actually very little 'rear overhang' compared to many other coaches, whether gas or diesel. The XCS' low stance gives it an even lower center of gravity compared to most any other coaches on the market, other than the 'twin' Forest River Legacy, or the Fleetwood Pace Arrow, or the Winnebago Forza, etc. This lower center of gravity actually helps the coach drive easier, have less movement when traveling, and less wind aggravation. It will get some push by passing semi's since it has a lighter weight of 26,000lbs gvwr, but that's simply a byproduct of the weight and size versus other larger and heavier commercial trucks, and/or the speed you are traveling and the speed of any wind, in general.
Now, I can't explain why you feel the way you do about your coach's wandering, but all of us have a different 'feel' to what driving is about, in general, and I might drive your coach and not feel the same way at all - I have some expectations about driving a 34' diesel motorcoach, and don't expect it to be like a cadillac(I owned an old smooth driving one years ago!), but I also don't expect it to drive like a gas RV, either - I've owned both.
You may find satisfaction by continuing to search for answers, but you may also find that it's just the way the coach handles, from the get go, and little will ever change that, unfortunately.
I've always commented on these forums that I've transported many gas and diesel coaches from the factories in N Indiana to dealers across the U.S., and have yet to hardly find ANY gas or diesel motorhome that handles and drives and is a smooth ride like my OWN Thor Palazzo. Those brands include Newmar, Fleetwood, Forest River, Coachmen, and the like, and include even the tag axle models.
The only exception was when I picked up a personally owned Palazzo 33.3 from a customer, to transport to a different location - it drove just like my own, and the owner had added nothing to change anything. It drove great.
I would add that the Fleetwood Pace Arrow comes close, and even some of the larger, longer tag axle coaches do, too, as they have a longer wheelbase, and the weight can seem to help keep them more stable.
Our Palazzo also averages about 12.4 without a toad, and 11.4 with... this is a great advantage as the other coaches I've transported got nothing close to that, especially the gas RVs and the longer, larger tag-axle coaches. Most of those were 8-9 at best.
I think if you adjust your tire PSI, you may find an immediate change in your perception of how it drives and handles. How fast you drive is going to play into the handling, too, as I don't drive the speed limit or try to drive in a hurry - I let traffic pass me versus passing them - that's just my driving style.
I hope you find the 'sweet spot' for your coach... : )
'14 Palazzo 33.3 bunkhouse 34' diesel
KingTailgater2 Dish HD, 100w/5a SOLAR, BlueOx
100,000+mi since '14 - US, Alaska, Canada.... formerly Georgia, then NorthCarolina, now Alabama : )