Questions.... is your water tank in an enclosed heated bay? And have you checked to make sure the water in the tank is not frozen? What sort of prepping have you done to your MH to get it through the winter. Do you have heat tape on the pipes? any insulation around the bottom of the MH?
As a recovering Minnesotan, I would suggest moving your motor home to someplace warmer and then figuring out where to put the space heater. ;D
Otherwise, I guess I would first open any cupboard doors that have pipes running through them. If the bays are heated, I would suggest not using the space heater as that would prevent the furnace from cycling on, which would prevent the heat from reaching the tanks. if they are not heated, try putting some type of drop light or heat light or something in the bay, and set your space heater near the open cupboards.
Good luck, it looks like spring is still a long way away up there.