Of the two class A's we've had and now the Seneca class C it is my opinion that none of them come from the factory with the best possible level of driveability.
On our first class A (gas 36') I added front and rear trac bars, Koni shocks and a Safe-T-Plus. We didn't keep the second class A (diesel 40') long enough for a full assessment or time enough to fix the handling issues--I did add 8 Bilstein shocks to the 8 air-bag chassis and a Safe-T-Plus, but it needed more. There's a 40+ pages of discussion over at IRV2.com with different ideas about how to make it better.
With our newest '08 Jayco Seneca (diesel 35') I've added front and rear trac bars, front sway bar (factory rear), Koni shocks, Safe-T-Plus, and Sumo Springs in place of the factory bump stops.
The reality is that the Seneca now drives and handles like a large SUV. Read; one finger driving possible.