Overflowing Toilet

This has been some trip! Tonight we're in Gallup and during dinner we hear a "thump" that sounds like it has come from outside. A visual check reveals nothing and I come back inside to my fork.
About this point, my wife who is sitting across from me, facing the bedroom, says "My God!" and races up the stairs to the toilet. Water is flowing down the stairs out of the bathroom. When I look, the toilet is full of water (clean) but pressing the foot pedal doesn't flush it away - it appears that the holding tank is full!
I race outside, turn off the water, and then use a pot to empty the water in the toilet down one of the sinks.
We've now tidied up but as we don''t have a waste connection here I'll have to wait until tomorrow to dump it.
(Fortunately we dumped yesterday and the waste tank got a good hosing out. There was nothing in it when we unhitched.)
So.. somehow water has got into the waste tank. No I did not connect the hose to the waste cleanout connection.
I have no idea how this could have happened and all suggestions are appreciated!
Many thanks!
Ian