I would believe the value entered on the placard. When we bought our last motorhome (2004 Beaver Monterey), the deal was contingent on Monaco swapping out the 20,000# rear axle for a 23,000# unit. A few weeks after we took delivery we went to the factory service center in Coburg, and watched them replace the axle.
Here's the relevant part: After they replaced the axle they replaced the placard with one that had the weight rating for the new axle. They appeared to take that placard data seriously.