Wobble in the steering

[Chamberpains]

What do you consider "highway" speeds. If it's 65+, you may be getting wobbles from pushing things too hard. You can get phantom or really hard to find causes of slight harmonic vibrations or wobbles at high speeds.

Another possibility that can cause strange wobbles in the steering can be if you use equal or balancers that get clumpy or stick or just come out of place temporarily. The tire temporarily goes way out of balance till it settles down or finds it's balance point again.

A 5th wheel being dry or not properly lubed can cause all kinds of steering problems and vibrations, clunks and creaks.

These are just some odd suggestions. Keep chasing the more important and possibly dangerous possibilities.