The other 3 jacks won’t hold up the camper unless it’s extremely heavy in back. Check it out by trying to raise the other front jack 99% chance the camper just tips to that corner.
Since it’s off the truck, you can raise the corner you need to by jacking up under the front corner on the bottom that sits in the bed.
I removed a front Jack this way. I just used a floor jack and a post with a good sized board on top to distribute the load on that corner. It seemed very stable not sketchy. Caveat, it was on level concrete.
Honestly, I’d first take the motor off and see if you can fix the shear pin, but if the worm gear is really corroded it may not move.
