Dry as a bone,…again

The holding tank is under/behind the bed. There is a wood panel next to the bed that I unscrewed and slid back so I can access the water pump, plumbing and actually watch the tank fill. We’re on our first summer with this rig, so I’m getting to know it. We bought it last fall and yes, the first time I filled the tank water spurted back out and I thought it was full. Nope. Trapped air. This time I had my wife watch it. She said she was sure it was filled, but I’ll double check next time.
We rarely dry camp, but there’s a really nice state campground on a lake nearby. Trouble is, it only has electric. It will be a pain to add more water storage. But, what else do I have to do. LOL