Thor put the sink in wrong, perhaps?

No offense to Thor owners, but as a manufacturer they are probably the best example of glitter and flash to cover inferior quality. I wouldn't say they did it wrong, I would say it's intentional. They probably knew the sink is a top mount, but buyers will get caught up in the "looks" of an under-mount sink without thinking about the long-term effects. I prefer function over form and would go with a top mount to mitigate further decay on the countertop. When you remove the sink, I would inspect the remaining top carefully to make sure there is no significant rot. Rv Countertops are small and if you are already removing the sink, it's a good chance to replace it relatively cheaply with something custom or better quality. Solid surface, butcher block, etc.