The optimum growing temperatures for tomatoes after transplanting are 65 night and 80 day. So, your day temps. are good. If the temp. at night does go to 40 in the greenhouse, it will slow the growth and they will be more suseptible to disease. If you could add just a small electric heater to keep the temp closer to optimum, you will have healthier plants. You could also consider a heating mat, placed under the flats/trays/pots which will keep the temp. around the plant somewhat higher than the GH.

Though 40 overnight is not an optimum temp for your tomatoesI have been growing mine with that night temp for years. You'll get sturdier stems and healthy plants as long as you don't leave them too wet.At the low night temps they will be susceptible to various diseases if too wet.In fact this past winter I kept some plants alive all winter at that temp and have been getting several tomatoes every day for months now.Good luck and enjoy,
PQ