Anything below 20 for more than a few hours. You need to run your furnace and not just rely on electric heat. Have a heated water hose or unhook your hose at night or fill your fresh water tank. I have had parts of my plumbing freeze with no problems as pex is very durable. I just tried to get things thawed as fast as possible.
2012 305sk traded, and 2010 F-350 power stroke with 220,000 miles. 2012 F-250 power stroke
2015 Grand Design 337 RLS