blown...mine was when the limit switch in the jack failed.
i suspect you have a bad battery (the one you tried first), or you have a bad power supply line from the trailer to the jack. not sure why two batteries work when one won't....12v is 12v, unless the one battery is not putting out 12v. your experience defies logic but can't argue with fact! so not sure what to say.
when he connected one battery, he connected one of two positive cables and one of two negative cables. my suspicion is one of negatives went to the ground buss bar and one went to the converter, and he used the one 