A sleepsack is a good start. We had to put our little guy in a set of thick footie pajamas and a snow suit with some winter socks for his bigger sisters. That kept everything except his nose warm and he learned to sleep with his face cuddled up with us to keep that warm.
A tent is kind of a bad way to have a kid sleep unless they are pretty wind proof. Little guys need some warmth or they will get sick on you at the drop of a hat. If you have a camper shell, I'd say that might be warmer than a tent if you did it up right. If you just have to be in the tent, then try to keep the wind out of it and off the outside of it if you can with a lean-to or something like that. Kids do pretty good till the temps get below about 35-40, then they tend to get issues with not sleeping well and sinus / colds etc. It can be done but you just have to find the right way to get there.
I second the feelings about a heater being problematic. They work to take the edge off but I won't sleep with one on.