All of my tanks, and some plumbing is exposed (not covered belly).
I used UltraHeat stick on tank heaters on my gray and black. They have a chart for what size heater depending on the tank capacity. For the exposed plumbing I used Easy Heat heat tape premade from Home Depot, and then covered it with foam insulation.
I didn't use my fresh water tank. Another Easy Heat on my fresh hose. I bought this one longer than my hose and wrapped it around the spigot, then covered the spigot with fiberglass insulation then a heavy trash bag.
With my trailer skirted with thin chipboard with 1" Styrofoam, I was good to -22*F.
Without anything (insulation, skirting), the fresh water hose shushed in the mid 20's.