I have researched every aspect and feature of all of the pop-top trailers like the Opus since mid Summer. Sold my Mission Overland Summit after 3 years and was planning my replacement.
Was focused on the Opus 13. Exceptional quality and features. You honestly can't go wrong. The killer for me was the Black Tank. I will never haul around or deal with one. Cassette toilet is closest I would chose.
Zero complaints about what I found about everything Opus. They are a copy of the original slide-out bed trailers from Australia. They seem to have done it correctly.
I share the sentiment, but this really shouldn't be a deal breaker. In my run of the mill Winnebago, the toilet is attached with two bolts. Remove those bolts, and you have access to the base plate. The plate is attached with 6-8 screws. Remove the screws, and you have a flat floor with a hole in it. Put whatever toilet you want overtop of the hole, and you're done. Forget about the black tank for as long as you own the trailer.
The black tank itself will weigh what... 20 or 30 pounds? Negligible. I carry more weight in beer.
You might want to read this thread on Reddit. Opus are made in China, which may or may not matter to you, but it’s something that not everyone knows. https://www.reddit.com/r/overlanding/s/OrCeUqEZvv