Had the same thing happen to us. I tried plunging but it ended up having to to take the trap out to fix it since the plunger made the rubber flapper in the trap get kinked up.
There's an access panel for the shower drain outside the bathroom on the left side of your sliding door leading into the bathroom next to the bed. You have to be a bit of a contortionist to get the trap out but it's doable . Took me about 20 minutes to get it apart straightened out and put back together while we were camping. . Works fine now.. Hope this helps.