In the Vehicle Editor, if I change the wheel locations or move the CG, the result isn’t what I expect. What am I doing wrong?

Probably nothing. If the Dimension Basis is set to Total Mass, there is a known
problem with these two dialogs. The problem is more obvious when working with
heavy trucks because the unsprung masses (wheels and axles) represent a greater
fraction of the total vehicle weight (when compared to passenger cars, for
example). The solution for moving the CG or editing wheel locations is to set
the Dimension Basis to Sprung Mass until the problem is fixed (in the next
release). You can use the Handling Properties dialog (click on the CG and choose
Handling…) to confirm the total weight on the front and rear suspensions.