[elhigh]

I think your best bet, considering you're looking at mostly highway cruising, is to minimize the load on your engine and skip any electrical drive or alternator delete mods. The car is proven in its current form, a long trip away from home is NOT the time or place to have a post-mod failure that didn't get found during the shakedown.

Tint your windows. Look at the local laws on the tinting limits, where I live you have to have a minimum of 35% visible light transmitted, and reflective films aren't allowed. Even so that can block a lot of heat coming in. If that works and the mod can be done, then you could consider pulley swaps that would slow down your AC.

Aside from that, I think that aero mods are the way to go. If something goes wrong you just tear them off, fold them up and stuff them into a convenient dumpster at the next potty stop, but the car will still be mechanically sound.

This is, to me, the least stress-inducing option, and vacations are about reducing stress.

Or as you suggested, rent another Prius. The money you save over the cost of gas in your regular car will cover at least part of the rental cost, and Prii are soooo quiet when cruising.