[travelin texans]

Even with hard piped to a sewer connection DO NOT leave the tank valves open all the time, especially the black tank. Leaving the black open will create a huge pile of hardened poop (poop pyramid) in that tank that's next to impossible to remove. They need to use plenty of water per flush & dump as needed. The grey tanks can be left open as they fill more quickly, but still a good idea to fill them occasionally then dump to rinse them out a bit.
As mentioned contact the local lp company & heat the water supply somehow.
You are better folks than I, wouldn't rent mine to anyone, especially someone new to RV lifestyle in the dead of winter.
Good luck with the experience!