Coachmen Leprechaun fresh water tank

I have a 2018 Coachmen Leoprechaun 21QB. Have taken many trips. Recently I filled my fresh water tank, waiting in the driveway for it to fill. When I heard the overflow water running onto the driveway, I looked underneath and it seem water was also splashing out of somewhere right by the overflow drain tube. Shut off the water and didn't think much of it. I did not use the presurre regulator like I usually do. Later in the day while on our trip I discovered that the water tank was completely empty. (drain valves were shut tight) Also, the black water tank that was emply when I left was now full of water.
As I drove to destination, I spent the whole time worrying about did I split my water tank of bust a pipe somewhere? I was so worried about a major repair needed. Arrived destination that day, and immediately went to work diagnosing. I filled water tank with a hose, until the overflow started running onto the ground and shut off hose. NO leaks anywhere, all normal. For 4 days now I have used the fresh water system (and I had drained black tank). Water pump on full time during the 4 days. Only thing I thought was odd is that when I filled tank at my destination, when it began to overflow, the overflow ran for a while onto ground after hose shut off- about 3-5 gallons flowed out. But all seems normal now. All plumbing, pump, heater, etc. work fine.
SO, what on earth happened at beginning of trip when fresh tank emptied from full, and black tank went from empty to full of water, and all seems perfect now??? Thank Dave