Turn on the water heater only when needed. Heat your water for dishes or cooking over the fire.
You might think that using Reflectix is only for keeping your RV cooler in the summer. It can also help insulate your RV in the winter and keep you from losing heat through your windows, which means you will burn more propane.
A vent insulator and skylight cover will also go a long way toward saving propane. These areas are another major source of heat loss.
Space heaters are another great way to add extra warmth and reduce your propane use. Of course, you will only be able to use these when you are connected to shore power or with your generator running.
If you have a gas-only water heater, you might want to switch to one that uses both gas and electricity.