As posted.... you should plan on supplemental heat if you will have an extended cold snap.
If you hot water tank is not electric and gas.... you will want propane for hot water and not use it all up on the furnace.
As Bob mentioned..... some higher-end coaches have heat pumps as part of their A/C units and they can provide a little heat from shore power when temps aren't extremely cold. They are not going to provide the heat of a furnace when temps are in the 30's or colder.
A ceramic heater or two is a good option to keep areas on the coach warmer without burning propane with the furnace all the time.
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