Robot system combined a mobile robot and a manipulator is called `mobile manipulator'. This paper discusses a dynamical modelling and compensation method of disturbance torques by the gravity for mobile manipulators travelling on a inclined surface, which steers by the velocity differences of the left and right wheels (PWS: Power Wheeled Steering). The Newton-Euler method is used for the modelling. PWS mobile manipulator has different characteristics from a floor-fixed manipulator, that is, it is nonholonomic. The velocity constraints integrate velocity and angular velocity errors of the mobile robot on a floor. The hand position and orientation of the mounted manipulator are determined based on the mobile robot's position and orientation including these errors. The errors do not disappear even after the dynamical vibration of the mobile manipulator settled. Therefore, to reduce the nonholonomic integrating errors, the velocity errors caused by gravity acceleration and resistances of rolling frictions should be reduced, especially when the mobile manipulator travels on the inclined surface. We proposes the compensation method, and the effects are confirmed by simulation experiments.