Inverter too small?

I have a 1000w cobra inverter i bought to power a small 5 cubic foot freezer that pulls 4 amps when running. The battery i use for the inverter is 105 amp marine deep cycle group 29.The inverter is connected to the battery with shortened battery jumper cables, at about 3 feet long, from the battery to the inverter. After about 5 hours the alarm on the inverter sounds and indicates low input voltage of less than 11 volts. The battery at startup was charged up showing 13 volts on my meter. Is the inverter too small or is the battery to small? I thought i could run the freezer with this setup for at least 18 hours. With this info i can forget running my cpap off this setup for overnight. Thanks!!!!