You need to have the diesel burner turned on to get unlimited hot water.
The electric portion of your Aqua-Hot produces roughly 5,000 btu of heat. The diesel burner is rated at 50,000 btu. By design the electric element is intended to keep the Aqua-Hot warm, provide hot water for a few dishes or a very quick shower. For unlimited hot water or to heat your coach, you need the diesel burner.
You are correct when the Newell is plugged into 30 amp shorepower. The 120 vac 1650 watt 5000 btu electric element is available to heat the Aqua Hot heater.
But, when the Newell is plugged into 50 amp shorepower or is running the gen, a second 240 vac, 4500 watt electric element automatically joins the 120 vac element giving vastly greater electric heating ability to a Newell install Aqua Hot heater.
Hence Forest is able to take a shower on electric only when on 50 amp shorepower.