Thanks for the replies.
1 - It's the bowl that's filling not the tank
2 - We have had some rain so we will check the black tank vent, though wouldn't that go into the holding tank first? So you're saying it filled up the black tank and then up into the bowl?
3 - We will also check the water pump, though since we didn't have any water in the fresh water tank it's not been used.
4 - The most likely option would be there is a leak into the bowl, though where is it coming from?? Was all that water just in the water lines?
5 - Could it possibly have come from the water heater? As I mentioned we didn't drain the water heater. Though if that's the source of the water wouldn't that mean we have two leaks - one from the water heater and another one into the bowl?
Again - thanks for any & all help!!