It may be, but unfortunately we do not have a breakdown of the specific components your 2021 Forest River Sierra Destination 399 LOFT Travel Trailer uses.
Instead you will need to take a look at the toilet installed in your trailer and look for the model number. The Replacement Wooden Toilet Seat # DOM83FR is confirmed to work with the Dometic 310 Series Part Timer Toilets. As long as that is what your trailer uses this is the correct replacement.
If you find you have a different model... view full answer...