I installed a Dometic 320 in my Itasca Meridian. It calls for 11 inches, I measured 10 1/2, and I still have clearance between the wall and the toilet. The only thing I see as being close is the angle of the lid as the toilet gets closer to the wall. You may have to put a latch on the wall behind the toilet to hold the seat/lid when in the upright position.
2007 Itasca Meridian 39K
350 HP Cat / Allison 6 Speed
1 Wife and 2 Dogs
960 watts of solar, 440 Amp hours AGM batteries