Check the ground connection. If it's via the mounting screws to the frame, the lights may work due to the small current but under motor load may not work. Loose screws and/or corrosion from sitting over the winter could result in a poor ground connection.
Might also consider a bad mini-breaker in the A-frame. Same thing, may be faulty and not work under high current draw. I would access the power wire to the jack and use a jumper cable or piece of wire directly to the battery pos. terminal and see what happens. If no improvement, try bypassing the switch with the jumper cable or piece of wire. Could also temporarily jumper across the mini-breaker and see what happens. If it's the breaker, they're pretty inexpensive and def. much cheaper than a new jack...
Last season, the day before we were due to go on our first camping trip of the year, our tongue jack also would not work. Even a breaker bar would not budge it. Luckily I had an old one in the shed that I was able to use.
Out of curiosity, what brand of jack and how old?
