I can't address the mating issue directly, but my 6.5L Peninsular drives a HD rebuilt 4L80E w/ Sun Coast Converters TC. To keep the RPM at best level (2,600, at the torque curve peak), I rarely get out of straight drive (3rd). My coach is slightly heavier than your rig at 8T.
You may find this thread helpful, expecially regarding gear ratios. Ed (MWrench) knows what he's talking about.
Anyway, it may be in your best interest to reconsider the 4L8xE.
'94 Barth 28' Breakaway M/H ("StaRV II") diesel pusher: Spartan chassis, aluminum birdcage construction. Peninsular/AMG 6.5L TD (230HP), 18:1, Phazer, non-wastgated turbo, hi-pop injectors, 4L80E (Sun Coast TC & rebuild, M-H Pan), Dana 80 (M-H Cover), Fluidampr, EGT, trans temp, boost gage. Honda EV-4010 gaso genset, furnace, roof air, stove, microwave/convection, 2-dr. 3-way reefer. KVH R5SL Satellite. Cruises 2, sleeps 4, carries 6, and parties 8 (parties 12 - tested).
Stand-ins are an '02 Cadillac Escalade AWD 6.0L and an '06 Toyota Sienna Limited.