My gosh this sounds familiar and I hope someone can post further the details.
I think one cause to check is that in heavy rains water can come in through the two grates on the exterior of the rig right behind the refrigerator. Then as the water drops down the rear of the fridge instead of exitting outside the water may fall back into the rig. The remedy was to add some flashing at or near the bottom of the fridge to deflect all the water to the exterior.
What you describe might be different from what is mentioned above only because it sounds like the water is coming in from above or another part of the refrigerator, but at least this is something to check. The problem with water leaks is that where you see the water coming in from is not necessarily where it originates. I hope you figure this one out real quick.
If I can find the forum post or other link (I believe I first read it in another forum) I will forward it on.
On edit: Here's the link about the flashing fix.
http://www.montanaowners.com/forums/...ad.php?t=29699
During my search I found other posts with problems such as the fridge vent causing the leak. The fridge vent exits the roof and is used for venting the heat created by the "absorption" process as part of the normal operation and the vent could leak especially if the vent cap is cracked, not sealed correctly or too low which allows water to bounce up and into the vent pipe. Unfortunately I think this means a visit to the roof.
Another search found that the seals at the slides may not position correctly and water can make its way inside.
These are just another few suggestions. Hopefully there are more to come.