It's the story of my life: you get what you pay for, maybe.
We had a Keystone 33F RV which came with many of the same issues you mentioned.
The EPDM rubber roof totally went after 15 years - that's probably normal.
We sold it to someone for $2K 2 years ago and I couldn't have been more thrilled.
It's tough to find a quality product even when paying a premium; I've stepped on that rake many times.
As far as trailers go, I have a theory that you have to look at the cultural norm for trailers in the area they are manufactured, because that norm sets the consumers expectations, and therefore the quality of the manufacturer's products.
In North America, off-roading with a trailer has never been the standard, but rather the very rare exception, so trailers are built for the pavement. Furthermore, the economic policies and social norms of "don't fix, replace" (ie. consumerism) encourage buying the latest & greatest every few years. And sure enough, this results in manufacturers making the cheapest crap they can sell, because they know that you'll never keep it long enough to really use it or run into serious issues. If people around here really cared about quality, they'd stop buying this junk, but that doesn't work when you're "trying to keep up with the Joneses."
In Australia, off-roading with a trailer has been the norm for decades, and while I have never visited, I get the impression (from meeting and working with Aussies) that folks there are a lot more old school, down to earth, and less likely to accept bullsh*t. Their trailers get properly used, so they have a lot more manufacturers building real quality units. I'm sure you can still find the same crap RV trailers in Australia, but you'll also have plenty of quality choices.
This is why I suggested the Opus as an option to someone earlier in this thread. That trailer is built solid, and there are plenty of videos on YouTube to prove it. The trailer doesn't have "off road" in its name, but it sure as hell is 100x more off-road oriented than just about every single "off-road" labelled trailer in North America. Of course, there are many more Aussie examples like the Opus.