What condition are all the tires in? I ask because I'd be tempted to get the four across the back alike and a matching pair on the front.
Most of what I've read says that your rear duals should be set at the same pressure across all four of them. But I saw something recently that made sense, which was... The Inners should never be at higher pressure than the Outers...
This relates to CapriRacer's comment: If the Inners are "bigger" (and higher pressure would make them effectively bigger) than the Outers, then your coach is riding on a narrower rear track than if the Outers have their share of the load.