If you have one battery, and are using lights and the furnace, you were lucky to get as much time as you did. With the battery as low as yours was the generator should have run around 12 hours to fully recharge it. If you have a multi-stage charger which would start charging soon at 14.4 volts, and then fall to 13.6 volts the charging time could have been 3 to 4 hours.
I suspect you have not been monitoring battery condition with a multi-meter (tested @ battery-terminals). Your battery most likely did not get recharged "fully" most times during the first camping season and while stored.
Your battery may be sulfated from misuse and has much lower capacity from when new.
