Hello,
I think that using a gas furnace/heater and/or the gas stove top would increase the chances of condensation, but we do not use the stove often since we do almost all of our cooking outside. During warm, humid conditions the AC does a great job of removing the moisture from inside the TC, so the only condensation I have noticed is on the outside of the windows, not the inside.
We do not have heated holding tanks as I do not think that was available our model year, and we have not added any heating element to them. We do winterize the unit by blowing out all lines and emptying all tanks and using antifreeze. However, we end up camping again and repeating the winterizing process a couple of times, but with the mostly mild winters we have down here, deep, long freezes are not that common.
The battery box is built for one, but I agree that there is plenty of space on that side of the TC behind the aluminum apron to make a larger battery box for two. It is something I have been thinking about doing myself. So far our boondocking trips have been short enough that one battery has been enough for our needs. We mostly camp with electricity available.
We have not insulated under the TC, but I have thought about it. We did install carpet to cover the floor and both the steps (a small square one that covers the water pump and the larger one that covers the fresh water holding tank). We used spray adhesive to attach the carpet to the step areas.