Fabric is really a key component to a pop up or RTT. Really is a lot of differing approaches. I was involved in some of AutoHome designs and the engineering of the fabric was taken very seriously not only the fabric but the colors.
I would steer away from the fabrics used in car covers, totally unsuitable, really different requirements. The reason why Sunbrella is so popular is that it is really durable and breathes well. The drawbacks are it isn't inherently waterproof and requires a DWR coating to be water resistant and Sunbrella can't take chaffing so it is OK for an application like a Westy pop-up but would suck for a folding RTT.
AutoHome has the scale so they can have the fabrics engineered for them. An example is the fabric on the Maggiolina, it is called Dralon and the work was done by Bayer in Germany. If you look at a light source through the fabric, you will see thousands of little holes, it is really just a mesh. You can actually feel a draft through the fabric in very high winds. That is why the tent works so well, holds in warmth, strips the water out of the air and breathes like crazy. You then stay dry because there is so little condensation. AutoHome uses different fabrics for each of their designs. For example, Dralon would be totally unsuitable for a folding RTT, but is perfect for the vertical walls of the Maggiolina.
So, what I'm getting at is that the fabric is integral to the design. The less fabric you use, the more it has to breathe. More vertical, less water resistance, more horizontal, more water resistance is needed. You get the rough idea. Once you get the design done, I think everyone can give more detailed advice.
Here is a little story to give you an idea of the fanatical attention to detail that AutoHome goes through. I was in Italy when they were designing the first of the aerodynamic Maggiolinas about 15 years ago. Gioseppi, the designer was asking me about how I like the new gray fabric. This is after a year of testing in the field. I loved it and told him he should use the new fabric on all the Maggiolinas. They previously used a blue fabric. He said that blue was used for the "quality of light". I was totally confused and asked him to clarify. He said, let's say you are in Morocco and just tired of the journey. You can climb into your tent, put on your music and the light inside the tent is soothing and you will be right in no time. You know, it is true. The blue fabric Maggiolinas are really cool and if I were going to go on an extended journey, I would specify their "expedition" blue.