I don't know what a MT is. I'm assuming a Class A. An engine swap would be, what?, $20,000 to $30,000? Doesn't seem likely that someone would do that. If it's a rear engine Class A, then it undoubtedly came with a diesel...and the current tranni.
Many Class A's, depending on length and weight, only get 7.5MPG to 9.5MPG with the average being 8.5MPG. As far as the previous owner only getting 7MPG, maybe he's a lead foot. You're pushing a large box down the road, and if you treat it like it's a hot rod, you'll probably only get 7MPG.
Maybe you should drive it like you drive your trucks for the greatest economy...I'll bet you get better then that 7MPG. 55MPH uses a lot less fuel then 65MPH.