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General straight line instability and instability over bumps and heaves is usually first an alignment issue (uneven toe, uneven caster (although more caster on one side can reduce pulling on cambered roads). Then a bad bushing somewhere - although a sway bar bushing would have the least effect of all of them. Bad busings will frequently cause toe to change dramatically under loading (braking or heave). A loose steering box counts as a bad bushing. A dead shock can also cause this problem (shocks these days die in bump, not rebound. When removed the rod will practically fall into the shock but it will take a reasonable amount of force to pull it back out - the jounce test is no longer a reliable indicator). Then uneven corner weights (although in a motorhome this would be a really bad design issue, but I had one at work where all the tanks and two 13KW Onans were on the same side, and it was a nightmare).
Wind instability with trucks is likely a combination of too little weight on the front axle and a lot of rear overhang. The pressure will act on the overhang and cause the coach to rotate about the rear axle, (weathervane, tail wagging the dog, etc) and the front tires aren't loaded enough to resist the motion well, especially as the axle unloads thru dips.
I have a 30 ft Beaver DP and I have this problem in spades. I did all the alignment tricks and when it gets windy I keep the fuel and water tanks full. I also added another water tank behind the front cap for ballast, and added a tube bumper filled with rebar.
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