There are a number of posts about this issue but not necessarily regarding your year/model. Use the Google search box at the top of the page and search for "shower replacement", "shower surround replacement", etc.
Winnebago can likely supply a replacement but I saw one post that mentioned a $1,200 cost. I'd call them first. A direct replacement from Winnebago shouldn't be too challenging to install since you wouldn't be re-configuring the plumbing. It might be worth the extra cost in ease of installation. You can also probably just buy the wall unit if your floor pan is in good shape.
Another, less expensive, alternative would be to check out aftermarket products like these:
Or even residential products that may fit, although getting them into place may be problematical.