When a mobile manipulator's wheel loses contact with the ground, tipping-over may occur, causing material damage, and in the worst case, it can put human lives in danger. The tip-over stability of wheeled mobile manipulators must not be overlooked at any stage of a mobile manipulator's life, starting from the design phase, continuing through the commissioning period and extending to the operational phase. Many tip-over stability criteria formulated throughout the years do not explicitly consider the normal wheel loads, with most of them relying on prescribed stability margins in terms of overturning moments. In these formulations, it is commonly argued that overturning will occur about one of the axes connecting adjacent manipulator's contact points with the ground. This claim may not always be valid and is certainly restrictive. Explicit expressions for the manipulator supporting forces provide the best insight into relevant affecting terms which contribute to the tip-over (in)stability. They also remove the necessity for thinking about which axis the manipulator could tip over and simultaneously enable the formulation of more intuitive stability margins and on-line tip-over prevention techniques. The present study presents a general dynamics modelling approach in the Newton--Euler framework using 6D vectors and gives normal wheel load equations in a typical 4-wheeled mobile manipulator negotiating a slope. The given expressions are expected to become standard in wheeled mobile manipulators and to provide a basis for effective tip-over stability criteria and tip-over avoidance techniques. Based on the presented results, specific improvements of the state-of-the-art criteria are discussed.