I could really use some advice and/or reassurance. I’m going camping next week with my 9 month old in a tent. It may drop to about 10 degrees c overnight and I’m wondering how cold is too cold? We’re planning to dress her in a long sleeved bodysuit, onesie and 3.5 TOG. I guess I’m just really concerned about something horrible happening overnight due to the weather. Thank you in advance.

Red Nose recommends that you dress baby according to the temperature of the surrounding environment.
Dress baby as you would dress yourself – comfortably warm, not hot or cold. A good way to check baby’s temperature is to feel the baby’s back or tummy, which should feel warm (don’t worry if baby’s hands and feet feel cool, this is normal).
https://rednose.org.au/article/room-temperature

If you are concerned about your baby getting cold, you can add extra clothing to your baby.
Red Nose information on room temperature is predominantly related to overheating & the risk from covering baby’s head & face.