If you drained the tank the pump should have no water in its supply line. The pump is in the trailer heated space so what is the worry about starting it? Even if it were frozen you would tell that it was not making that pump sound and would shut off the switch, right? My guess is that if the traps did not split your only real worry might be an outside shower. When I went to pink up the camper I found there was no water in the system except for the outside shower hose after I had opened the low point drains. Perhaps if I had opened that shower all would have been dry, leaving only the traps to pink.