You've pretty much hit the nail on the head.
I can say having owned both 5.3 and 6.2 vehicles, there's definitely a noticeable difference. The 5.3 is perfectly adequate, and with as little as you drive it on the highway is perfect for your use case, especially if MPGs are a focus (Not that either of them are that efficient considering you're driving a house on wheels). The 6.2 has a lot more "get up" on the highway especially, and for towing (which I do a lot) it's an absolute must. On the flip side, I only see 14-15 mpg around town. So it's a tradeoff.
I considered a diesel for the long distance MPG benefits, but my commute daily is only 2 miles, and I read that is hell for diesel engines because it doesn't come close to operating temperature- diesels are meant to be run longer.
Thanks MPlunk. I was thinking the 5.3 might make sense for me too. Is the 5.3 overall pretty reliable or is this one of the engines that has had a lot of concerns?