That sounds like a restricted drain between the sink/shower and the tank. Being that it's so new, I'd definitely be letting the dealer handle that one.
If you want to tackle it yourself, I'd get a borescope camera that connects to your phone and stick it down the drains and see if you can see what's clogging it up.
Salem 29RKSS Pushing a GMC Sierra 2500HD!
Gotta go campin!