It voids the warranty, but I assume you mean to fit it on an RV bed, or something similar, it's doable. Just be careful to keep the cuts straight, and then re-sew the outer covering and it should work. I've resized foam cushions before, this is just an extra thick one made of specialized foam. Besides, it's not even 200 bucks, a custom RV mattress can cost a small fortune, so it would be worth it, if that's what you had in mind.
If you removed the cover, which means you'd have to cut the material, and cut the mattress, and then mend the cover. It is a Thick solid slab of foam. It isn't impossible but I don't know if I'd recommend it.