I need HELP!!!: Water Issue

On our first camping trip with this trailer I had an issue with water leaking or overflowing. We travelled to our first camp site with 1/3 full fresh water tank. After getting to our site we hooked up the connection to the city water supply line. We were there for 2 nights with no issues. On the third day we travelled to our next camp site, again with still 1/3 full fresh water tank. On arrival we again hooked up to city water supply. After 3 days we woke up to start packing up and heading home. As I was making my way around the trailer to start disconnecting thinks I heard running water and noticed that the filler cap for the fresh water tank was leaking. Checking the holding tank monitor I noticed that the grey water tank was full. Not surprised as I had not drained it since day one. I also noticed that the fresh water tank was now full as well even though I had not filled it since we had originally set off at 1/3 full. I could hear water running still and after investigation I found water leaking into the pass through storage area from somewhere above. I turned off the water supply for the city water connection and pulled the drain valve for the grey water to drain some of it out and then mopped up the water in the pass through storage area with towels.

Now as far as I know the fresh water and grey water systems are separate so my questions are; how did my fresh water tank get filled to overflowing? Is it possible that the grey water tank being full somehow back filled the fresh water tank? If so I now have a contaminated fresh water tank! I have bought some tank cleaning solution and am about to treat the fresh water tank and flush the entire system as best I can but I am struggling to understand how this could have happened. Any insight would be appreciated.