HANDLING PROBLMESThe Freightliner XC chassis, and the Discovery motorhome, normally is one fine-handling motorhome.   If everything is adjusted properly, you should be able to travel down the highway with one hand on the wheel - not that this is the recommended driving style!
Here are some things to check if your D is not handling satisfactorily:
First, make sure that the weight on your front and rear axle is approximately the same percentage of Gross Axle Weight Rating (GAWR).   For most of us, this means that we travel with our fresh water tank full.
Second, make sure that your tire pressure matches your load on each axle.   Consult your Michelin book for this info.
Third, check your wheelbase on each side and make sure your coach is not "dog tracking".   Be sure to use a metal tape or chain due to the length involved.
Fourth, check your ride height.   Per Ron Anders, Technical Sales Manager at Freightliner Custom Chassis Corp., the correct front ride height should be 10 inches and the rear should be 9-3/4 inches.   [Be sure your coach is on level ground when you check this].   This sets the axle caster and driveline angles to the correct settings.   Measure the distance from the underside of the frame rail to the centerline of lower shock mount bolt nearest to the height control valve.   You can either adjust the ride height yourself or have Freightliner do it.   To adjust it yourself, the older coaches [up to mid 98s] have an arm with a slot and a 7/16 in nut at the center of it.  As you change the adjustment you are either adding or releasing air in the air bag behind the front wheels.   There is an adjustment for each wheel.  The newer coaches have two 7/16 in bolts in the frame.  What this actually does is changes the caster and gives it more lead.
Last, you may need a front-end alignment, which costs about $100.   However, if your coach is still relatively new, you may be able to convince Fleetwood to pay for this.