Calculating time constant for storage-tank water heater
Best I can tell, your math is right. I've never done thermal R-C calculations before, but what I got agrees with your numbers. Here's a plot of the temperature over 10 days if it started at 140°F and ambient temperature is 70°F:



Basically what this is saying is there is a lot of energy in water. It takes about 8 BTU to heat a gallon of water only one degree F. It only takes about 0.2 BTU to heat the same volume of air one degree F. It takes about 400 times more energy to heat water than air per unit volume. Since the heat is so concentrated in the water's volume, it's easier to insulate and keep warm because there is less surface area.

Here's the thing though, this model assumes a sphere of liquid completely separated by insulation from the environment. In reality, you're going to have pipes, other components and the water itself which will also dissipate heat into the environment affecting the final temperature.

Here's a related discussion on a Thermos bottle