An engine swap for my landyacht

I am getting closer to finding a donor to be my landyacht. It is a 1992 extended econoline with a 7.3L diesel. I have not bought it yet but want to ask you some questions first. I have heard that this gets 12-14 mpg on the freeway. Is that correct? What kind of mileage would it get with a turbo?

Is there another engine that would fit in this van and would give better mileage? I have already been looking into the 4BT. Will that mate with the E4OD? I would be a little worried because it is about half the displacement of the 7.3L My reason for wanting to change engines is that the present engine is a little tired and it would be cheaper to put in a used engine with a turbo on it rather than buying a new turbo and installing it on the engine.