If it is just dropping down below freezing just at night and not for any extended periods, the tanks should be fine over night. I have been down to 20 deg at night, and back in the upper 40s during the day. No issues. What I do is fill the fresh water tank, and disconnect the hose so there is no chance of freezing it up, and utilize the pump at night. If the heat is on in the TT, there is little chance of the plumbing inside will freeze, but there could be very cold spots depending on the TT design, and possibly cold enough to freeze. So I leave a few cabinet doors ajar where the plumbing is running through them. In the colder months we always have electric sites, so I leave the water heater on (electric mode). With our plumbing layout, if I get up to use the bathroom in the middle of the night, I turn on the kitchen facets (it is at end of the water distribution pipes in my HTT) for a moment to move a little water through the pipes. An empty tank (grey water) or nearly empty tank will more likely to freeze than a full tank. So we do not stay connected to the sewer system. As a full or partially full tank has a lot of thermal mass and will freeze up very slowly in comparison. One other thing, I would not leave the sewer hose connected if the temps are going to be below freezing for extended periods. What will happen, warm moist sewer gas will rise up and condense on the flexible RV sewer hose walls and freeze, with time it could freeze solid, or when it is time to move on it is frozen in whatever shape it is sitting on the ground. If they are sitting in one spot all winter, putting an incandescent light bulb under the black tank will radiate enough heat to keep the tank from freezing. If parked long enough it could be worth adding some insulation board around the base of TT to help keep the floor warmer.