We are full timers and have the GD 378MBS. We have a room GD calls "Lounge". We have multiple names for it - Lounge, office, cat room, guest bedroom. An additional half bath would have been nice, but you have to pick and choose. We wanted to stay under 38' in length but ended up settling for almost 42'.
I think you meant either the 397th or the 399th - both toy haulers. We liked the 351m and thought long and hard, but ultimately decided we wanted a regular 5th wheel as we think they feel a lot more like a home.
Are you planning on doing any boondocking? Or primarily just RV parks?