Is there a reason to have batteries off while on shore or generator?
But as to whether you can or not depends on the converter. From my little knowledge some can, some cant and some that can supply 12v without battery connected are limited. You can run lights etc but not power high loads like jacks.
Without brand & model of your converter or if factory the year model of your camper, not a lot of information. Just throwing some guesses out. I would verify first if your converter is designed to use with battery out of loop. 'Sounds' like yours did previously? Short of failure, check breakers and fuses. Ours has a separate inverter breaker, IF yours does, other than ensuring its on, verify (with AC power disconnected!) its working. Long shot but breakers fail.
I honestly don't know if our converter would supply 12v or not directly if I turn off batteries while on shore. Ive just never tried. Only our jacks are connected directly to battery (though they wont lift long without a charge source).
And Welcome to the forum!