I'd think you want the tanks as close to the left side for flushing? The shorter the distance the better wouldn't it be? Maybe it doesn't matter, I don't know. I guess you at least want the black tank directly under the toilet, that must be the most important thing isn't it?
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio