It is not hard to remove the underbelly material just time consuming, remove all the screws on the sides remove the support braces and it all comes down with the foil insulation behind it after you do that take a look and see where it is coming from ,I have a 2012 3402rl same basic floor plan as yours and my main grey tank and black tank are side by side(Grey is driver side black is pass side) under the front belly cover under the hallway/bedroom area the Kitchen holding tank is located well under the kitchen belly pan get a good flashlight to get a good look ,maybe you split a seam or a valve broke loose but you will see
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