li10up,
I'm no expert, but if you only drank one and a half bottles of Gatorade and a third of your Camelbak, for a 5 hour ride, it sounds like you were massively under-hydrated to me - especially given the amount you sweat.
I don't know how big the bottles were, or how big the Camelbak (I'm assuming about a 2 litre capacity), but given the Texan heat, you should be drinking AT LEAST 500-750ml of fluid an hour. Probably nearer the 750ml mark (about the size of a large water bottle). That should have been ALL the contents of your Camelbank, the two Gatorade and more besides!
Dehydration can be the biggest cause of fatigue.
From what you said, you sweat LOTS, therefore you need to drink enough to replenish the moisture you're losing (without going to the other extreme and over-hydrating).
The Gatorade site has a fluid loss calculator, which might be helpful.
http://www.gatorade.com/hydration/fl...ss_calculator/.
I honestly think that your hydration may have had a big part to play in your fatigue. And that's speaking from experience - as someone who never used to drink enough, and would always start to feel "flat" halfway through the ride. As soon as started to pay attention to the fluids I was taking on board (along with the fuel), my fatigue disappeared.