Hi Chris,
I am no expert on tank heaters, but our rig came with them so I know something about them. The kind we have seem to be like the ones you pointed out from Camping World. They are heating pads that adhere to the underside of the gray and black tanks, I assume a peel-&-stick application. Under our rig, I can feel the heater patch, one on the bottom of each of our two waste tanks, covered by a full coverage thin silver insulation blanket. Our tank heaters operate on 12 volts with the power to them controlled by a manual switch located inside the motor home. Our switch happens to be located in our bathroom.
The fresh water tank, the hot water tank, all fresh hot and cold water plumbing, and even the waste plumbing to a point is supposed to be inside the motor home, protected from freezing temperatures. There are always exceptions. One example will be the outside shower. Of coarse this assumes you keep your interior at a reasonably comfortable temperature. If not, then winterizing the rig is a "Must".
Because of the toxic nature of the waste tanks, it is a wonderful thing they are mounted outside. Also consider the waste is gravity fed and it is also a great utilization of under-belly space. Unfortunately the waste tanks and their valves are most vulnerable to freezing which is why tank heaters and heating tape on exterior valves and associated plumbing becomes necessary for freezing weather use. But for freezing temp storage, the best process to follow is standard winterizing.
If camping with hook-ups in freezing temps, I think you are spot-on considering a heated fresh water inlet hose.