Is it cheaper to use a window unit other 2 rooftops ?

I would always choose to use the rooftop AC as in our coach it is vented so the cold air is sent to all areas of the coach. With a window unit you will be getting a huge blast of cold air right at it's location and hardly anything off in another room. We have found that ours cools really well and we actually seldom need the thermostat set any lower than 80 and we have never used both units at the same time. We however have never stayed in an extreme heat area in the summer (like the southern states)so we might use both if we stayed south.