Your drinking water can freeze causing you to having no usable water in the morning, or it can potentially crack the container it is in. If the temp is right near 0 and you have a large quantity of water it might not fully freeze, but this is something to take into consideration. I camped at 17F (-8C) one time and most of our water froze.
+1 to putting warm water bottles in the sleeping bag before you get in. Otherwise a near freezing bag can take a while to warm up and is not very comfortable.
A lot of heat is lost through conduction to the ground. With the roof top tent the ground isn't an issue, but when you lay on a sleeping bag it compresses and looses most of its insulation value so a pad/mattress with a decent R value is important.
Avoid sweating. Some people recommend exercise to warm up right before bed, but make sure you don't build up a sweat. Similarly if you have too much insulation (sleeping bag, clothes) you can sweat in your sleeping bag, and then later be really cold due to the excess moisture.