A new motorhome should ride and track correctly BEFORE it rolls off of the showroom floor. A thorough test drive will reveal any inadequacies which are, at that point, the dealers problem. New C's should not require thousands in additional equipment to insure a decent ride. If they do you are buying the wrong MH. :S
I suspect the OP's coach needs some adjustment to its tire pressure and perhaps an alignment. As noted a little more castor can often work wonders on the handling. A trip to the scales is warranted here so that the tires can be brought to the proper psi. At 55 psi (in the Michelin LTX M/S 2's), all around our 24' C (E-350), rides smooth and quiet with no handling issues whatsoever. :C