Being a "used" PC, I wonder if a previous owner wired an inverter into the rig somewhere, and it may be running things at 110 volts, things you don't realize are on. Some inverters all by themselves with nothing running can draw a lot of current.
Also check around the rig inside and out like an outdoor storage compartment for example. You might have lights left on. We have a light in our Sanicon area that can easily get over-looked, but yours being a 2005, you don't have a Sanicon, hence no light.