[JRTJH]

Many of the water lines run under the floor, but you also have many of them that run above the floor, inside cupboards, against the outside wall. Typically these water lines are the first to freeze and water stops flowing. There may be hot water in the water heater, but there is cold water in the water lines (until you turn on the faucet and water runs long enough to flow from the water heater to the faucet). So, a hot water line, laying adjacent to the outside wall of the trailer, inside an unheated cupboard, under the kitchen sink will freeze even if the inside air temperature next to the thermostat is 72F.

I'd suggest you turn the furnace up to a temperature as warm as you can manage, open all the cupboard/cabinet doors, turn on the faucets and let things thaw. Once that happens, you'll see water flowing from the faucets. Until then, the more heat you apply the faster the pipes will thaw.