Is it actually 5-7 volts at the battery verified with an external voltmeter? If so, then about the only thing that can cause this is cables , a bad battery, or a weak battery coupled with a dead short (and even then, there is usually a bit of drama with it).
If you have not actually verified the battery voltage with an external volt meter, and are simply reading the gauge (which I doubt, it doesn't go that low), then please check it promptly.
If you know the cables are not to blame, and you have verified the voltage drop at the battery, you may simply have to throw a battery at it. I hate suggesting throwing money at a problem even though you confirmed the battery is good, but I don't believe there is anything else it could be.