On all the other floor plans the fresh water tank is installed under the bed area, so in conditioned space. On the 24M this would not fit, this the duct work.
As for no thermostatic control, as long as the furnace is working then you should be fine. This is how most Class A's do it and how we do it on all our gas Class C's. All you're trying to do is keep the fresh water tank above freezing. So if you're warm and toasty inside (or just warm) the tanks should be fine. The thing I don't like about the 12V pad heaters is when dry camping it drains batteries pretty quickly.
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