I'd say if the Allison is working fine then just maintain it properly and drive the heck out of it. If it gives up then drop it out and bolt in a manual. No sense spending money on it if it works fine now. I always see breakdowns as a chance to improve things.
'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