We had a different issue with our 21 3121rl/legacy, the toilet flange was not properly sealed/seated. A bathroom smell would increase as the black tank filled. Thorough investigation of basement/plumbing showed a slow leak around the toilet flange to black tank downpipe (ours was a threaded flange), with poor placement of sealant in the joint.
I removed/replaced the flange with better quality sealant properly applied, and we have not seen a leak since or had the smell.
