I'm a bit discouraged by the quality of the responses written above so I'll try my best to give you good information.
First, as someone has already mentioned, you are looking at an '03. Unless that coach was impeccably maintained, I'd be very surprised if you didn't have suspension issues that could be easily adjusted / fixed. How many miles were on the coach? How old were the tires?
All the posters telling you that you are wrong and don't know how to steer a coach are projecting and could be completely wrong. They may be right as well, but I wouldn't assume you are the problem. I would assume the coach is the problem.
Don't consider the road crown too much either. If the suspension is properly adjusted, it should take into account the road crown as well.
If you really like this coach and want to buy it, ask the seller if you can take it to a freightliner shop and have them look at it. There are dozens of adjustments and improvements they can make that will make a huge difference. Four corner alignment, fix loose connections, ride height, tire pressures, etc. All could be factors.
As far as Spartan vs. Freightliner is concerned, it really depends on the components on the chassis and not so much the frame manufacturer. Both bolt on other companies' suspensions to their frames. Spartan makes some very good chassis today in their higher end units. I can't speak for the lower end. I'd imagine that the upper end Freightliners are very good as well.
Finally, I can drive my coach with two fingers without wandering at all in most moderate wind conditions. Windy isn't much harder either. Unless you've driven a well adjusted coach, you just can't believe what's possible. Most folks have no frame of reference.