Pretty well anything you would be interested in would have come with a shower originally...
Let me add though that until 1974 (for most models) there were no "grey" tanks in AS trailers. The grey tank holds used wash water. If you are in an RV park, you very likely will have a drain connection, so you won't care if you have a grey tank or not.
But if you want to camp where you can't dump used water on the ground, you will need a grey water tank to hold it.
If it's to be, it's up to me.