The space heater should be enough to protect the interior piping as long as you open cabinet doors, etc. to allow the warm air to get into the spaces where the piping is located. If there is piping in the enclosed underbelly that is normally kept warm by a duct from the furnace you probably should run the furnace instead of the space heater. The waste tanks should be OK with only a couple of gallons in them. Temps rising to the 50's in the day will warm things back up so the water in the tanks might not freeze at all. Same goes for the water heater since it is mostly inside the heated RV and not likely to freeze even if shut off.