A lot of my recent professional experience is in spray foam insulation, energy efficient construction methods, wood stoves and off-grid solar. Here are my thoughts:
1) Think of your bus as a system made up of sub-systems. The summer comfort system includes insulation, shading, ventilation and (perhaps) mechanical cooling.
2) Close cell spray foam is the best insulation, bar none.
3) Shading is critical. If your bus is out in the sun, it will absorb a tremendous amount of heat and it will take hours to cool it off before you can sleep comfortably. Conversely, if your bus is well shaded it will never get much warmer inside than ambient temperature. (There will be some interior heat gain from your fridge, cooking, bathing and people's warm bodies being in there. That's nothing compared to the heat gain from the sun hitting an un-shaded bus!)
4) After shading comes ventilation. I like Fantastic Fans!
5) If you'll be in a hot zone most of the time, do a roof raise even if you don't otherwise want one. Before air conditioning, homes were built with high ceilings so the hottest air could rise ABOVE where your head will be. You'd be astonished how much difference that makes!
Then, install your Fantastic Fan(s) in the ceiling to get that hottest interior air out of there!
6) There's enough room on the roof to install a large PV system. That not only provides shade - it provides enough power to run a small air conditioner for a few hours a day if needed!
Some other observations:
I don't like Trex decking in general, and specifically not for vehicle decks. It's heavy as hell (bad for your cg) and it absorbs (and hence radiates) a lot of heat. It can easily get too hot to walk on!
I wouldn't worry about a thermal break between the bus body and the mounting legs of the deck. That's a small area, it's highly unlikely you'd ever notice the trivial amount of heat or cold conducted into the body. (You are insulating on the inside, right? That's all the thermal break you need!) Much more important to get a strong mount!
On that note, I don't like the pipe mount in the pictures. There is not adequate side-to-side or front-to-back rigidity in that design. If you drive much, that deck is eventually coming off the bus, and not in an orderly fashion.
My awning idea is to use 80% shade cloth to make awnings that will shade the entire bus. It's relatively cheap, not terribly heavy and made for UV resistance/high winds. I've used this stuff in greenhouses and over part of an outdoor garden, and like it.
I think it would be a great idea to build a canopy out of this stuff that would be large enough to shade an entire bus while working on the conversion!
Hope this helps everyone!