We had the same issue arrived at in the same way as the OP. Had a Swift and knew precisely the weight of all the kit, then bought a Chausson which, with the same kit in it, should have been fine for payload, but of course, when we weighed it it was overweight. We stripped it back to as near ex-factory as we could, and it was over the specified weight by 150kg. Which, interestingly, was still just about within the 5% tolerance that most manufacturers quote in their specs. Easily uprated, but a pain in the proverbial!