Well, I've never done HHH, but since I live near that area and it's currently in the 100's and hotter than hell up here, I would say you gotta wear what's most comfortable. If you're comfy wearing sleeveless, just lather on the sunscreen. I would think a short sleeve jersey would be best; who cares about weird tan lines when you're out there for 5-7 hours. Comfort is key. You should be fine with just your 2 water bottles if you plan on stopping and refilling at each station (from what I've heard from my friends who've done HHH). If you plan on not stopping very much- use the camelbak. Again- do what's best for you. For me, I drink far more with a camelbak (and don't mind it on my back) and during a race like that you NEED to stay super hydrated.
Again, it's just my $.02. I've never done that ride, but have done plenty of 5 hour rides in that area under those conditions...
Plus, you're from Houston; you KNOW heat and humidity. You'll do GREAT! Have a great ride.