Sorry that your contact with Dometic ended in failure.
I think the problem with the toilet is a leak between the upper and lower halves. You'll have to find some way to separate them and then re-seal them. Once the toilet is fastened to the floor I imagine that there can be a lot of torque on that joint as users move around on the upper half.
Fortunately a new toilet is not all that expensive. If I remember correctly a new 310 is around $150.
Rick (K8RWG), Joy, and Daffy (the spoiled Calico cat)
2014 Rockwood 8315BSS
2016 F250 SC LWB 4x4 6.2 3.73
Reese Dual Cam