Help! One of my jacks won't retract or extend!

Some people are just plain hard headed. No photo would convince them. They would blame Photoshop. All you need to do is start with a level camper on all 4 jacks. Retract any jack and the jack on the diagonal will be off the ground. I would not expect this to be stable. I would not get in the camper and walk around. I would even be concerned about a strong wind. With the camper on 3 jacks, you can prop it up with a stack of pallets for example and then remove and replace the bad jack.