The rv rear is most likely a dana 60 or 70 way to heavy for you cruiser build.
Look at the rear on the postal if it has no bolts in the rear cover it is an 8.75 which means you can change out the gears easy, the entire center section comes out gears and all.
I just bought a open carrier 2.91 complete off here for $100 shipped, so don't let the gear ratio scare you.