Our 2 year old 300 did similar, along with a serious odor that couldn't be resolved. Recieved a new 300 under Dometic warranty within 4 days of initial call. No hassle at all. They included a new flange seal and hardware. Super easy repair.
As stated above, Dometic will replace and ship a new style 300 free of charge to owners with early design, or in-warranty, leaking commodes.
Gather your pottie serial number and a pic of the RV serial number plate.
The old 300 has the serial number located beneath the flush pedal. The newer units are on the back.
Newer units can also fail--rare to see.
There was no discussion about 310 models, so cannot advise. I've read that early on some 310s were shipped due to no new 300s available?
If you peer down into the opening on the upper backside of the pottie and down where the inner and outer shells meet, if anything other than clean plastic is visible, the seal has failed. An intractable noxious oder is typical also. There is no "approved" repair for internal seal failure.
If your unit proves to have failed, do not tip it while handling. Keep it vertical. There may be nasty fluids inside the shell that will pour out if turned sideways. It's very light and easy to R&R.
Be sure city and pump fresh water are off before removing the supply line.