Well, given the OP's outlook I would do 24v in a heartbeat. Regarding the Flexmax, I thought that was the whole dingdarn point: 24v from solar.
Don't forget you still need 12v for the water pump (unless you buy a 24v pump), possibly 12v for the fridge, and absolutely 12v for the emergency TT brakes. Yes, P Tuna will have kittens over the unbalanced nature of this.
@Mena: I view you as the poster child for expensive batteries. To me, 24v batteries implies super-premium, and I remain to be convinced these things last any longer. Since the OP's not going to do 24v batts, it's a "mute" point, as they often say.