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. 

Way2roll is spot on, the problem is THOR!  They buy out everyone they can and then immediately start cost cutting measures with complete disregard to quality.  Your best solution is to buy a top mount sink to fit the space and make it right yourself.

And in my opinion, if you are replacing the top, might be a good time to consider how much sink you really need. A round or smaller sink might provide more counter space and a single handle faucet in the corner would get you even more. I find that between the 2, I would always opt for more counter space over a larger sink. Seems like RVs never have enough counter space. And almost all new RVs come with a single handle gooseneck faucet with flexible sprayer neck. They save a lot of space. 

Transolid Brushed Nickel/Black Single Handle Kitchen Faucet with Sprayer PF7509A at Lowes.com

as for the skink its self there are a lot that can be top or bottom, I would tend towards using it as a bottom mount, if any moisture is getting in it is a sealant fault rather than how they mounted the sink.  you can have the same issues with a top mount sink if it isn't sealed up good enough and maintained.  

My sinks are recessed like that and there is a cover that can go over the sink that sits flush with countertop. 

Funny, you posted this issue 2 years ago in another thread and pretty much got the same advice. If you are going to replace the sink you should replace the countertop. You mentioned in your 2 year old thread the particle board under the laminate was already compromised and swelling. That was 2 years ago. I doubt you'll get a good seal just by replacing the sink.