You say you were hooked up to campground water. Perhaps the spigot was shut off where you were hooked up to the water.
When, you say, the toilet didn't flush could you still get water to come out of the faucets in the sinks?
Did you have water in the fresh water holding tank? Was your on-board electric water pump turned on?
By the way those gauges to indicate the level in the holding tanks never work right. We haven't looked at our gauges for ten years now. We determine the fullness of our tanks by how much we have used the toilet and ran water in the shower and sinks.