Load test a bank of 6 AGM house batteries.

Ok A series of questions. and the results
Your load tester is (I hope) a good 12 volt unit with adjustable load

If the AGM's are six volts paired. you test each pair as though it was a single 220 amp hour 12 volt. This means 660 amps most load testers max out around 400-500 so it's not the best test

If they are 12 volt.. Again you test each battery by itself.

WHY: the load tester can't pull enough current to test 'em all at once.

(Just got done doing that with mine a couple months ago by the way I can hit 450 amps but consider if it's still Green. it's Green.

First just check the voltage is 12.6+ Check right on the battery terminals to verify this is not a wiring issue.

Yes? Check how much the voltage drops on the battery terminals when cranking the generator.

No? Charge the batteries. Minimum 14.4 volts on the batteries for 4 hours.

If your genset won't crank w/ 6 AGM batteries you may have more than one problem. test the charge as they sit. To load test batteries, I have a set-up with an inverter and a small electric heater. I test the batteries individually. The inverter and heater draw about 13 amps from the battery. I let them run until the battery is down to 50% or 12.06 volts. Then I add up the amp hours and check the result against 50% of the batteries rated capacity. I replace a battery if it falls under 80%. Not a throughly scientific test, but it gives me a good idea of the amp hours a battery will give me during actual camping conditions.

Your boost switch CONNECTS all batteries together so I think it would help to crank your generator. I have always believed the only way to test a group of batteries is to disconnect them from each other. If tied together for the load test, you will usually find out the condition of the worst battery. By taking them individually you can find out if only one is bad. If you have more than one battery that tests weak, it is usually advised to replace all of the batteries.