Well, that's a bummer! But also not that impossible to deal, depending on how you are set and how you want to deal with the problem.
I'm a big dummy and have always done lots of dIY repairs, so that is one I know about but I'm not sure I would do it again!
The thing about this is there is not likely to be a ready made replacement to buy off the shelf as it has a bit of age and not that much call for getting 20 year old cabinets tops!
But to see what you "might" want, there are the parts drawings here:
This for the 2004 322E model:
https://www.winnebago.com/Files/File...04/4wf322e.pdf
This for the 322R model:
https://www.winnebago.com/Files/File...04/4wf322r.pdf
But they do both show up on page 101, depending which model you have, so some checking for stock might find something???
I would do a full search for Winnebago parts online and check a bunch of them if wanting to go that way, after getting the correct part number. You might also try just a search for the specific part number to see if you could get lucky!
But if more DIY inclined, I might look at how bowed it is and look at using a belt sander to level it and then do an overlay of new material. This requires removing the faucets and likely some trim but not terribly difficult if the damage is just the material swollen/bowed some. Is it paricle board that expands so much when wet? It does than require adding some surface to blend the whole into the RV and not very likely to find exact matches.
Have you done any laminate cabinet tops at any point? It becomes much easier as you gain experience and tools but it may also be better to have the top made and you do the less complex part of setting it in place.
Best of luck as it can get tricky. Mostly requires lots of dedication!