This is just a thought, but why not just get yourself a front and rear hitch carrier? I have a shortbed camper on my (shortbed) truck and I'd much rather have the additional 18" of floor room than that small amount of space in the bed. I don't know if your truck is a dually or not, but if it is, there are some very large truck campers that have way more storage than a smaller shortbed camper and you'd be able to use at least a front hitch carrier to carry more stuff.
If you have something specific you need the bed space for, then maybe this isn't the best idea, but if there's a way you can have both the longbed camper and the room to carry the stuff you need, I say go for that rather than the shortbed camper just to get that little bit of space back.