Extension cords and power strips are not recommended for use with your heater. Power strips are not ignition protected, nor moisture resistant, and should never be placed in the engine compartment. Improperly sized extension cords can cause voltage drop to occur, and could even present a fire hazard.
If you must use an extension cord, make sure it is heavy-duty to handle the electrical load of the heater, and rated for the environment you intend to use it (i.e. outdoor, wet conditions). Extension cords should not be used in the engine space unless also rated for fuel/oil resistance and 105°C temperatures, as ABYC standards dictate.