The electric jack on my previous travel trailer was wired into the battery terminal directly with it's own fuse. If there was no battery, the jack wire was just dangling in air. If this is the case with your's in question, then you could jump the jack easily with a pair of jumper cables, which would run only the jack.
Most electric jacks have a manual override. The rubber seal on the top pops off and then there is an end for a heavy duty Allen Wrench style jack to insert so it can manually be raised and lowered. At least mine did.
Check it out, either one of these methods will work to operate the tongue jack.
