I believe all RV furnaces are 12 volt so I am not sure what you are referring to. If you are plugged into 120 volts the furnace will still operate off the 12 volt system. The 120 volts will just recharge the battery for the 12 volt.
If you have an AC unit that has a heating option, that would run off the 120 volts.
I have attached a manual for the Dometic DF unit. If you have any more details please post.
Retired electronic technician (45 years in the field)
2017 Greyhawk 29W
solar & many other mods
wife (maybe I should have given her top billing)