In the case of the Onan 4000 there is another reason to run it periodically, these Onan RV generators have brush style generators, the carbon brushes run in slip rings on the rotor, if left sitting, particularly in a humid environment tarnish builds up on these slip rings. When tarnish builds up it increases the resistance of the circuit, which then in turn tends to burn out the voltage regulator on the combined voltage regulator / control circuit board. (older models had a separate circuit board for voltage regulator and control board).
