Hey Shane, thanks so much for your response. I initially considered trying to friction weld the attachments myself, but after I saw the prices I decided to send the toilet back.
As far as the "C" model. I can't find it anywhere. Looking on the Dometic website, I can't even find any evidence that the model C existed.
I tried looking at the scamp website as you suggested before, but they are out of stock.
I ended up going with Ardemco.
https://ardemco.com/sealand-31237110...let-with-tank/
Someone from the Facebook group suggested them, so I contacted them. I HAD to speak with someone on the phone to ask for the spin welding I wanted added to the bottom, and sealing the vacuum hole as well.
Regardless, including shipping it cost around $820 which is more than I wanted to spend, but hopefully it will be worth it in the long run.
Thanks again for the response. Hopefully this can help others in the future.