Dead in the water, what to check?

I have a 1996 Coachman Santara. It has been sitting idle for just over a year. I went out to crank it this weekend and it is dead, nothing. All the batteries were dead so I put a good one in place of the engine battery but still nothing. If I put the key in and turn it, I get no lights or anything. If I try to turn on the headlights I get nothing. Are there some simple things to check that could be causing it? I don't really know anything about them, a friend is giving it to me. He said it worked fine when it was parked. Any advice would be greatly appreciated.