We have a 2020 Bounder 35K but I assume this can happen on any model. Our drain was very slow from the time we bought it a year ago.  I sought out causes but could not find anything specific.  We had tried non corrosive drain cleaner but that did nothing. We also used a small plunger and that worked for a little while but the problem returned.  Eventually I went looking for the plumbing and found it behind the drawers next to the shower under the Washer/Dryer. (or rough in if you don't have one). Remove the drawers and you will see the setup in the attached photo. 

The culprit was the white Hepv0 Sanitary Valve. It is essentially a duck bill one way rubber valve that allows water out but closes off to prevent tank smell from coming back up through.  This item has to be installed with the arrows pointed towards the floor.  It was sideways and probably had been that way since new. It is held in place by two plastic compression collars that are only hand tight so very easy to remove. I took it apart and cleaned out a bit of sludge and then reinstalled it correctly.  I then used a garden hose and flushed out the shower drain to ensure everything was cleaned out upstream of the valve.  That fixed the problem and the drain has worked great ever since.  Hope this helps..... Steve