I had the same issue, as have many others. Instead of replacing the 300 with another 300 I upgraded to the Model 320... https://www.dometic.com/enus/America...tdataid=107354
This is a real toilet size and is made of porcelain, not plastic. I actually replaced the seat with a real toilet seat that has the slow close feature so it does not slam.
If you decide to do this, just make sure it fits. It is also 18 inches tall, which is nice also.
Be careful when you remove your Model 300. The reason you get the odor is that the seal between the top half of the toilet and the base failed and has allowed stinky liquid to seep in and it is puddled in there. Do not tip when you remove or you may be in for a surprise