I don't have a bus yet, just an old Class A motorhome. It has external undercarriage holding tanks and a freshwater tank under the bed (above the floor). I use the RV in the winter for ski trips with outside temperatures dipping into the low 20's (F). Here's what I did as a stopgap measure, and it seems to work okay:
I installed two small electric tank heaters, one for each tank, black and grey water:
Then I applied Versafoam to the underside and sides of the tanks. (I couldn't access the tops without removal.):
The heaters use a lot of electricity, even the small ones. So I only turn them on at need. The insulation works pretty well to keep the liquid from freezing overnight. I might run the heaters while boondocking only an hour or less at a time, with genset or engine running, just long enough to get the temperature up, then turn off for several hours. If connected to shore power they can be left on, as they have an internal thermostat, provided that there is fluid in the tanks.
The freshwater tank and supply lines, and greywater sink traps are inside the cabin above the floor, so cabin heat keeps them above freezing. When I'm not using the RV, I keep it connected to shore power and use an oil-filled space heater (from Walmart) on low setting to keep internal temperatures above freezing and to minimize humidity buildup.
At the end of each trip I empty the holding tanks at an RV dumpout, so I don't need to worry about the holding tanks freezing during storage.
This method has worked so far, with zero incidents of freeze damage.