Yes, the water is collecting above the coroplast underbelly enclosure you had mentioned (Now I know what its called).
The water seems to collect at the back 1/3 or so area of the RV.
We put a few small holes in the coroplast, this drains some water continuously, and then every couple of months, remove a few screws, bend the coroplast downwards, and that drains most of the rest of it, but then over time it re-collects.
We live in the unit full time, and the RV never moves, so no water from tires.
We do not use the freshwater tank at all, and its empty, and the water pump is off.
Could it still be the freshwater tank? or a connection to the freshwater tank?
And if so, would you know where the freshwater tank is, in relation to the kitchen sink?
The water that does collect is very clear, and does not have any sort of odor.
We live on Vancouver Island, and it rains here all of the time, as in pretty much every day, from late October into early April, and we have always thought that it was likely the rain getting in somewhere near the back of the RV. We first noticed the water accumulation last November, and am unsure if it was there prior to that,
We've looked and looked for entry points for the rainwater, but have been unable to locate any.
Also we've noticed that the water collects above the coroplast, but is below the tin-foil like layer that runs along the top of the coroplast (I'm not sure what the tin-foil thing is called)? I think that its used to trap heat?