I don't know anything RV specific - do the same you would test driving any used car/truck. What do you mean brakes seem good? Did you get it up to 35 mph and slam on the brakes? Take it onto an expressway and see how it feels at 70mph.
How about the tires (and wheels for that matter) ... Rub the side of the tire - see if cracks have been filled in with shoe polish (I saw a neighbor doing this on a 1970s boat trailer he was selling.)
I guess the other thing to ask, you're buying something 40 years old - what are your expectations - do you expect it to drive like a 2015 model?