You need to find out what the system is doing. First step is a GOOD ammeter (30 or 50 A) and voltmeter (15V or so) - NOT radio shack stuff.

With everything off put ammeter in series with battery. Lite bulb is good for this test also - no lite = no current therefore you have nothing left on (brake lites etc)

Next install ammeter between generator and battery. Install voltmeter across battery. Crank up and read amps at idle - rev to 1500 and see what amps are. Read volts also. After start amps should be high say 50 or so and should die off as battery recharges. Volts should be 2.2 to 2.5 x battery cells ie about 7 for a six.
As amps die off volts may rise a little.

Now turn on loads one at a time and watch amps and volts.

Now for book work. You need to know capacity of generator. Once you have rated capacity then you can decide if you are overloading it OR you have other problems - quite likely adjust regulator.

While doing all this a quick n dirty check for bad terminals or wire is to check temperature with fingertip -- hot means bad wire (undersized) or a bad connection.

Dan Bentler