NO 12v supply is almost instant-
I do recall reading warnings on using high draw, like jacks, on converter only/no battery as it 'could' damage converter. The only time I could see using converter 12v without battery is IF I had battery failure- otherwise no reason to take the battery out of loop.
As to whether or not 12v should work without battery? now I do not know, what I thought I knew was wrong. But I guess my question would be why?
My old converter didn't, battery had to be in place. New one (same camper) does supply 12v with no battery in place which I verified this morning by disconnecting battery cables. Unless the charger and converter sections are separate (doubtful) old converter along with overcharging failed to supply 12v to panel...