It’s a bit complicated, but, if you do your own conversion, the vague law suggests that you do not need seatbelts on the rear seats.
However, your insurance company will almost certainly insist that every passenger has a seat belt.
The Police will probably also insist that all passengers are wearing a seatbelt.
So the reality is, you need seatbelts.
If you are on a campsite hookup, a small electric heater is best.
For off-grid camping, a proper vented heater, such as the Propex 2000 is good. It’s expensive, but very good. I have one.