If you have heat in the living area, I would not worry about 19º, same as Dononan, But with coach heat on, I'v been to zero, (I have remote thermometer by water tank). Thats when driving, not parked.
Thanks for the "Degree" thing, how soon we (I) forget, I also had a Sinclar, then Atari 400 next Atari 800, was "Past President of S.P.A.C.E. (St. Paul Atari Computer Enthuiast). Oh the good old days of "Hand entering programs"
Bruce