I see you have no responses so far and I should say that I am not familiar with your coach specifically. It would help to clarify your problem situation a little more first. You say water is pouring through the tankless system.....do you mean water is leaking inside at the water in/out connections; or the physical heater core itself is leaking somewhere else?
I am not familiar with Winter in Texas, but I think you can get below freezing in some parts, so if the leak is from the heater core it might be possible that you did not Winterize correctly (or at all?). I think with tankless systems (where you do not leave the systems energized – full time), Winterizing means either blow out the water lines with an air compressor or pump in RV pink antifreeze.