While the reservoir needs full fluid to operate the jacks properly, a low level of hydraulic fluid just makes the pump run without much effect, or jacks go only partially down. If the fluid is low, you should also be seeing/hearing the "jacks down" status (and alarm if the engine is running), because low fluid level is how that is detected.
The jack controller has a couple of interlock inputs that would prevent it from operating, but they are wired by the coach builder, not Power Gear. It's conceivable that National RV hooked up an off-level sensor to one of them, but I've not ever heard of it. The 2002 Dolphin I had did not have it. Typically it's just ignition off and parking brake set.
Former owner of 2004 American Tradition
Home is in the Ocala Nat'l Forest near Ocala, FL