About three years ago we saw a B with a pull down (murphy type)bed in the rear. It may be the Great West van that a link is posted to in one of the responses. It did not seem very popular at this RV show and I think (may be wrong) that it was not a model that was continued.
Yes, it is a good idea as it gives the option of making up a full bed with sheets in the rear and folding it out of the way. The power sofa/bed is often used with a sleep sack or sleeping bag and no sheets. We don't have the power sofa/bed. We just leave the bed made up while we are traveling, use the front if we are eating in, and don't really need the space where the bed is to be kept open.