3/4" flooring will look fantastic in that RV. I would air on the side of caution and use 1/4" to 1/2" subfloor to lay down first, this way you can ensure a level floor for the install. One thing I would like to point out from previous expereince with installing hardwood for a client, in RV's, hardwood will expand and contract more so than in a home. Avoid mastic glues as they will not allow the floor to move at ALL! Nailing them is going to limit expansion and contraction as well, but certainly a better option than the glue.
Although I really like the look of the real hardwood, I try to point customers in the direction of the engineered, just solely on the basis that engineered flooring can be layed as "floating" floors, meaning, no nails, no glue.
In your case with an older RV, you should be fine to use the real thing, but adding that extra layer of subflooring will assist in keeping the floor from over expansion or contraction!
Good luck and be sure to take lots of pics for us!