Water expands as it freezes, that is what causes piping to break, not water pressure. Even with the pump off there is still water in the lines and the pump.
In a situation where there is a good chance of freezing, open the low point drain and drain the lines or use what ever means is available to maintain temperatures above 32 degrees.
Many RVs have ductwork that directs heat from the furnace to the wet bay and holding tanks, check with your manufacturer.
Turning the pump off and opening the faucets is no guarantee the remaining water will not freeze and burst the pipes or the pump.
