Hi Robert,
You aren't alone with the steering problems you are describing. I have a '97 Continental I bought 5 years ago with the same problem, and I expect there are others. I have yet to solve the problem but some of what I have found out from my experience may be of some help.
So far I've had the coach aligned (no help) had the king pins and bushings replaced (no help) ride height adjusted (no help) replaced tires (no help) weighed on all four corners and adjusted tire pressures (no help), aligned again (still no help or improvement). I did take the coach into Henderson's in Grants Pass, OR last fall. Nice folks and seem to know what they are doing, but they insist that the only way to correct the problem is adding about $6K to $7K worth of aftermarket parts. Really!? This much in aftermarket parts to correct a problem it should not have left the factory with and perhaps didn't. Yes Tiger Track bars would probably reduce some of the swaying, and perhaps Koni shock would provide a smoother ride, but these really weren't my complaints. My complaints were similar to yours. The rig just won't stay in the lane at highway speed without constant attention and steering input.
I'm thinking there must be a simpler solution, so I started reading up on what I could find out about alignment and steering.
First thing I discovered from reading is that most alignment shops cannot solve drive-ability problems. They don't have the technical experience or knowhow for this. They do not test drive vehicles either before or after performing an alignment to assess driving problems, they simply "adjust to specifications" and return the vehicle to the customer. This may or may not solve your problem, but that really isn't their concern.
Second thing I discovered is that the three basic alignment adjustments are camber, caster, and toe-in, and each has a different function and effect on steering. Toe-in is basically to compensate for the caster angle and reduce tire scrub and wear. Since caster on the Westport axles installed on our rigs isn't adjustable anyway, toe-in gets set between 1/8" and 1/4" degree. Incorrect toe-in can result in the vehicle pulling to one side, and rapid or uneven tire wear. Caster angle though is another story and is very important. Caster angle is what provides the force for returning the steering wheels to center from a turn. The force actually comes from the weight of the vehicle, acting against the caster angle of the steering spindles to provide this centering force.
You can test for correct or sufficient caster angle very simply, as I finally did. Find an open space such as a large paring lot where you have room to maneuver your rig safely. While traveling forward a low speed, turn the steering wheel 90 degrees or so to one side or the other setting up a turn. Then let go of the wheel. It should quickly return to center resulting in your rig traveling approximately straight forward. This is exactly what you are used to in your car. It is so commonly expected, that when it doesn't happen you probably don't even recognize the problem and just return the steering wheel to center yourself. If your rig is like mine and the steering wheel doesn't return to center, then the rig just continues to turn in a circle after you release the steering wheel. I never recognized that this was specifically the problem, and that I was always returning the steering wheel to center from any turn. While this isn't a huge problem at low speeds, at highway speed it results in constantly needing to return the steering wheel to center, the problem I was having, and the problem I think you are describing.
So unless you have a low tire, wildly off toe-in, or extremely low ride height, if the steering wheel will not return to center on its own from a turn, then you either have something binding in the steering system, or insufficient caster angle. Something binding (king pin, thrust bearing, rod end, etc.) or incorrect caster angle are both alignment shop issues. The shop should be inspecting for any loose or binding parts prior to making any adjustments. Any adjustments should be expected to minimize tire wear, eliminate pulling to either side, and sufficient centering force to return the steering wheel to center when released. If all these conditions aren't met, then the alignment work hasn't been properly completed by the shop. Simply setting the alignment adjustments to the recommended specification may not result in meeting all the conditions, and if this is the case the shop may need to make further adjustments.
There are "caster shims" available to make adjustments to caster angle. Safari alignment specifications call for 4 degrees positive caster. The alignment shops I've been in assure me that I have 4 degrees positive caster. The Westport axle service manual suggests 3 to 5 degrees positive caster, so apparently the 4 degrees recommended by Safari is just the starting point.
I think my starting point next spring when I get the rig back out of storage is that I will be much more specific about my steering problem when trying to select my next alignment shop. Telling the alignment shop that the vehicle wanders, or is difficult to drive, simply isn't specific enough. I think perhaps something more like, "Steering does not return to center on its own from a turn, and I refuse to pay for any alignment work that doesn't correct this problem" might be specific enough to get the point across. Either that or the shop will refuse to accept the work which would only confirm that it is one of the many shops that can only adjust the alignment to "spec" and really has no ability to solve any drive-ability problems. If the later is the case, I've just saved myself a lot of time and frustration, and I can continue searching for a shop that has the expertise to solve the problem.
Hopeful some of my experience will be of some help to you.
Glenn
'97 Continental 4006/Cat 3126