The only good way you can get this kind power level is the power inverter which it will convert the 120 V AC to 12 V DC for your furance.
Get a inverter that rated for 5 amp I know quite few RV dealership will have something on hand without issue otherwise you can use the battery charger but I don't really receomened it due somecase it will have unstable voltage and chopped DC which it may make odd noise on your furance.
Some electronique store will have this on hand so check it around.
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