I'm guessing you are wanting to move the mounts on the camper FORWARD, not back, if the struts are shorter than the ones recommended for your camper/truck combination. Is that right?
If so, here are the things to consider;
Is there enough material built into the camper to mount in the new location? Lance provided me with a template of where I could mount my mounts. You should call them to get the specifics.
Lance says the shocks need to be at the same angle your windshield is at. That means, standing back and looking at the side of your truck with the camper on, the strut will be parallel with the windshield. I would think that the shorter struts, with the bracket moved forward to get them to reach, is going to make the strut stand more upright than slanted back like your windshield.
Lance says that the struts need to be placed so that there is enough length for the strut to move BOTH in and out. That means the strut's piston, the shiny part, is not fully extended, but about 3" or so compressed.
Here is a photo of the struts on my truck camper;
Of course, from the front, the shock is not even with the windshield, but instead slant out since the camper is wider than the truck at the mounting points. Here is a photo of that angle; from the front;
I recommend you get the right shock for your job. Sure, it's pricey, but I know for a fact; the single biggest improvement for ride comfort has been these struts. As long as the US highways have those expansion joints and potholes, the struts are the one thing that makes the ride and the piloting of the rig much more manageable.