That's the thing, they don't exactly say. We looked at a 33ft unit with the thermal pkg at the dealer it had coroplast over the tanks and we were told there was a heating vent under there that comes on when we use the furnace. I believe the isulation is just the pink fluffy stuff. The tanks are not individually heated. Our current trailer doesn't have the underbelly enclosed and I do find the floors a tad cold when we first open up in the spring and during the our trip last thanksgiving. When the furnace is running though it is pretty toasty. Most of our camping is during the summer months. My husband does a good job checking our current trailer for possible holes the mice can get in and blocking those up. He is worried that you can't see the enclosed space and now you have a sealed, warm area, with fluffy stuff for mice to make their nest. I will look into that stuf-it stuff. Also not sure about the durability of that darco tarp like material that comes on the trailer without the thermal pkg. Thanks for your help handye9.