In many full size vans and trucks, V8s provide better gas mileage than V6s. Given that you’re looking at a large van that will be loaded with stuff (certainly not close to it’s max weight, I wouldn’t think), I think the V8 will prove to be necessary, and not that much of a guzzler. Indeed, when you reach the mountains, having a V8 will certainly come in quite handy. The mileage might be around 20-25 mpg on the highway, an equivalent vehicle with a V6 would get around the same, just with less power.