We have the heated underbelly as well. As long as you are running the furnace your tanks should be ok. I called dutchman when we bought this and they did not recommend camping below freezing, I'm sure I was told that for liability reasons.
The water line I am talking about, sticks down through the underbelly so you can drain the water out of your lines. There should be two of them, just in front of the sewer connection.
Haven't had any issues with our water heater, it is the standard 6 gallon one, I know it gets the water hot enough to really burn your skin, so you don't have to use as much. I wonder if something may be wrong with yours. From what I can tell if you did want more hot water there is probably enough room for a 10 gallon unit to go in its place., but I am not sure if that's possible.
As for the great stuff, I tacked up the foam flooring first, then peeled the edges back and emptied two cans into the voids that were left. The foam flooring material was bought at BJ's, it was with the mattress. I have seen it at Home Depot and lowes for quite a bit more money.
Hope you enjoy your new rig! This is our first, we have been tent campers for years.