I am assuming you are looking for an automatic OD for an RV and the short answer to that question is there is no OD that will bolt on to any BB Mopar.

The easiest thing I could think of for you to investigate would be to go the Gear Vendor OD route. Pricey unit but would probably require few mods to your RV...probably just a shortened driveshaft would be the worst. This is a unit that bolts on to your existing 727 (replaces tail shaft) and adds the overdrive. They seem to run in the ~3K range IIRC.