How am I going to re-attach this jack?

Image did not come through. We'll need more info too, like make & age of camper. Many here have dealt with what your going through.

post your image here.

http://photoposting.is-great.net/?i=1

Photo here of the jack on a '99 Fleetwood Elkhorn 9.5. There is some split lumber in there. Thanks!

It’s not the end of the world. You’ll get it fixed. No camper hasn’t goofed.

Bummer dude!
At least it’s tin sided, hopefully you can remove the siding easily and then you can get down to work.
Good luck!

Of course you already know that will all have to come out and be completely rebuilt. Elkhorn's are noted for their water intrusion. It doesn't look like there is much to attach it to is my point. I see some dark wood, rot maybe? It looks like the mounting plate came of with the Jack the bolts are still mounted to it so that tells me of rot damage or did you tear it off or did it fall off. I'm guessing by the location on the truck you might have just pushed it off while mounting or did it just come that way? All guesses without many more pics of the damage and some detailed info. Your in the rite place though.
Joe

The problem is that the wood is rotted so the jack has nothing to attach to....the "repair kit" is to peel back the siding and remove/replace the rotted wood. Chances are very high this is not the only place you have rot., Good news is that aluminum siding is easier to repair/replace than filon. The bad news is that jack itself seems to be damaged now and not sure what that will take to rebuild.

What you'll find is that camper has been leaking at various places for years and this was the first place that rotted out bad enough to be a problem. If you want to rebuild, check the link in my sig for what you are likely in for. i wouldn't discourage you from repairing this be be aware of what you might be getting into.
If you look at the link in my sig to the Lance rebuild, this is excatly how it started out....the RF jack by the propane compartment gave out.

I'd be looking at welded steel at each corner, running the entire length of the camper. 2X2 angle at the bottom, with 2X2 at each corner, at least a foot long. Gussets where you can fit them.

This does not look good. Everything can be rebuilt, but first you need to start disassembly, then you can assess the damage. It is frequently a lot more work than anticipated.

For a general overview, go to the Truck Camper University at the top of the TC section
https://forums.trailerlife.com/index.cfm/fuseaction/thread/tid/25966285.cfm
(or rv.net, or whatever flavor of the Open Roads Forum you use)
Scroll down to "Camper Structural Repairs". There you will find a lot of good reading material.

If the only damaged part of your Atwood jack itself is the cover, you may find a 3d-printed replacement. The jacks have been discontinued and parts are hard to come by.

I would build a corner pocket out of aluminum or steel to cradle the corner and mount the jack to. If you were in my neighborhood, I could help you with the fabrication