If you are staying in one place for most of the winter, under skirting would help with keeping your tiny home warm and tanks protected. There are many forms of under skirting available: plywood, heavy-duty plastic, foam and cement blocks. I’m sure there are others that I’ve missed. One thing that may cause a problem is using straw or hay bales. They attract rodents; get moldy and are hard to remove when they break down.
Holding tanks and lines are always a main concern for people. Holding tank manufacturers claim that a typical heated and enclosed underbelly will only protect from freezing down to about 14-20 ° F. Holding tank heaters are available in 12-volt DC and 120-volt AC models and are thermostatically controlled to turn on below 44° F and turn off at 64° F. Testing on these products have shown that exposed holding tanks were safe to -11°F and enclosed tanks could withstand even colder temperatures.
Some options for your water lines would be to install some heated tape You can also install foam pipe covers to help keep your piping nice and warm as well.
Last but not least, if you are staying in one spot for the winter months that has hookups and you want to protect your fresh water supply, you could install a heated hose. These are thermostatically controlled that will turn on when the temperatures drop to 45° F and turn off at 57° F.
Living “Tiny” is an adventure. An adventure worth living!
Holding tank heaters
Foam pipe covers