The key point here is:
USE the battery disconnect switch.
Connect when you want it to charge.
DISconnect when it is just sitting.
If you do that correctly, then they should be charging any time that AC power is present, regardless of the source.
EVERY RV owner should have an inexpensive voltmeter so you can check and be sure instead of just guessing.
And I agree that a half hour probably isn't enough; an hour would be better.....both for the batteries and the generator.....but the gen will not go up in smoke if you don't do it "exactly right".