I can't help with the conversion but we have a 2007 Winnebago Sightseer 35J with bunks, we've had it since 2006 and it's served us very well. Our boys are now 24 and 21, the don't go with us very often, but if they do it includes our sons gf now. My youngest likes to sleep in the top bunk, but we use the bottom bunk for storage. If the two of them go they use the jack-knife sofa. We also take a couple of those zero gravity lounge chairs that work well, plus one cot. We can set those up down the main aisle and it almost works out better then the bunks now.
All that to say I wouldn't build anything until you see the need.