I was just out in the mountains this past weekend and we slept in our RTT at about 25 degrees. We have a Autohome Maggiolina and we use flannel sheets and a down comforter. Neither the wife or I were cold at all during the night. I would feel very confident taking it down to the teens without hesitation. I did however just pick up a Mr. Buddy heater in case it was too cold. My plan was to pop it up in the tent for a bit before we got up there to warm it up. But as it turned out, we didn't need it. Our tent has about a 2 inch foam pad as the base and a small layer of egg crate foam on top of that. Slept in drawers and t-shirt. Throw on a skull cap and some good dry socks and you will be golden. I can pass on one other tip that we learned when we were still sleeping at ground level in a tent. When you or your partner roll around during the night, that movement usually sucks the cold outside air right into your sleeping bag with you. Kinda sucks. We found that getting a couple of those quilted military poncho liners and stuffing them in the bag with you takes up a lot of that airspace and limits that rush of air into your cozy warm bag. At 20 degrees, things like this do help. Enjoy!!
