Do you really want to put in an engine from a "free to good home" RV?  That engine is likely going to be as worthless as the rest of the RV.  You'll end up having to rebuild it - just like you would your current engine and tranny.
Most RVs sit for a long time before their house becomes that trashed.  That means their engine has been ignored for a long time.  And if they're offering it for free, that means no one wants it - so even more no ones are going to want it after you remove the engine and tranny.
It sounds like you are willing to do a lot of work to get an inexpensive engine.  Have you considered looking for a newer van or truck at a junkyard that was in an accident but the engine was undamaged and retrofit your RV to fit it?  It's a lot of work, but you could walk away with a better engine for not much money if you do the work yourself.  Or could you just rebuild the engine and tranny you already have?  Or look for a van from the same model years as your chassis at a junkyard and grab it's engine and tranny?