That is what I would call a good question. This is my thinking on the subject.
If the tanks are electrically heated and you don't camp in winter they are of no use whatever. However, if you do, they are, I suppose, a portentially good feature. Think of it as heating pads applied to the outside of the various tanks to keep the contents from freezing. My own oppinion is that "heated holding tanks" sounds impressive to a potential buyer and is a good sales pitch, but not much more than that. When folks start asking the same types of questions as questions as you have posed...
Generally speaking, the only application I can think of where electrically heated tanks would be of use to me is one where:
A) I was hooked up to shore power. (If the heaters are 12 volt, how long do you think your battery will last, especially if you are running your furnace? If 120, then you need to be plugged in.)
B) I was using my internal water supply. (If it is cold enough for long enough to freeze the contents of your tanks, then most campgrounds will have turned off the water to protect the water lines, and, your hose will be frozen solid anyway.)
C) I was either on a sewer site or had a dump station available to dispose of all that nice warm effluent before heading home.
If, on the other hand, your tanks are heated by warn air from your furnace being circulated around them in an enclosed space, then you are still going to have to deal with A, B & C above. (Furnace blowers also take a serious toll on batteries.)
Happy Camping! ///// Richard D.
2006 4x4 Ford 250 SD / 2007 Flagstaff 827 FLS
One very patient wife and one furry child who travels with us. Forty-two years of trailering and camping, and I still have a blast.