Do we need to turn off the water when we leave for an extended amount of time to prevent this from happening again?
Yes.. Lucky it didn't fill up on ya!
Not really sure what would fill the grey tank when hooked up however?
Might need to test it by putting some water in the fresh water tank and turning on the water pump and see if it 'cycles' on/off without anything being used water wise?
It might take awhile for that to happen if over 4 days, the grey tank didn't overflow?
I've seen a couple of campers in full hookup sites with a nice waterfall feature added to it when the owners left the water turned on and something broke inside... I turned off their water when I saw it, but they didn't get back for hours..
Good luck! Mitch