It really does add a ton of usable space, and is a very pleasant setting for drinking tea every morning.
As a bed, it's really pretty uncomfortable. I had trouble sleeping the first couple nights. Now I'm used to it and I sleep just fine, but it's nowhere near as comfortable as a bed. I actually like this, because I felt like my bed was a bit too luxurious, and I want to train myself to sleep on any surface. In a couple months I may try sleeping on the tatamis with no futon.
This is the only tatami that's light enough for the RV, and only that store sells them. I bought this futon with the Organic cotton option. It's equivalent to the lightest non-organic one they have. If you don't care about organic (I mainly care because I don't want pesticides and flame retardants in my bed), you could get a thicker one.
Before I had the futon, I was temporarily using this. Its springiness makes it harder to roll up in the morning, but it was thinner and slightly more comfortable. Not authentic in any way, though. If you go that route, this 3" one might be a perfect level of comfort.
To set this whole thing up, just throw away the mattress and cut down the tatamis to size. You don't have to be super accurate because they do squish a little bit. The process of cutting them is a huge messy disaster, especially trying to seal them back up afterwards. I did the whole thing with my pocketknife, which was not the best choice. When you do it, cut the last couple inches off first to practice, so that you'll understand what you're getting into before making the final cut. It will take you a couple hours.
Last, to get a tea table, search ebay for Gong fu and get the biggest one you can find. It's Chinese, not Japanese, but it's a really great design and very practical. I got lucky and found an old one with inlaid brass and copper.
If I didn't cover everything, feel free to ask questions..