Tent Trailer Heater I Have A Small Hardtop Tent Trailer With Propane Heater.Does Anyone Know How Long A 12 Volt Battery Would Last?

I have a small hardtop tent trailer with propane heater.Does anyone know how long a 12 volt battery would last? - tent trailer heater

Ibought they used and no fan manual.The should run for office, as I do. told.The owners have lost address.It thermostat.john s and running.