I have camped many times in our HTT in the mid to low 20's. Never been an issue. But I have never used it in the teens.
Do disconnect your hose, drain it, ideally coil it up and put it in a cargo hold. It will freeze solid if you do not, and will take all day to thaw out.
I suspect you have an enclosed underbelly with heat?? I like to fill the FW tank full, but we rarely ever have hook up, and I have an open underbelly. I would say you want the tank at least 1/3 full for night and morning use. If your going to see the teens, and have an enclosed heated underbelly you should be fine. More water, more thermal mass, longer it will take to freeze, so no harm in fill it greater than 1/2 full.
Do you have tank heaters? If so, run them if it gets below 25ish.
Will you have sewer connection? If so, disconnect from it at night. Otherwise your flexible hose will fill with frost (condensation from the sewer system).
We tend to keep the cabinets ajar that have water pipes in them. This will allow a little heat to migrate back into some of those areas.
Run your furnace, as it will heat your underbelly (if you have a heated underbelly)
Hot water heater: as long as it is turned on you will be fine. The tankless units have freeze protection, the tanked unit has a thermostat that will just cycle the water heater on as needed.
Stay warm