||You don't state what year is your RV, if the engine is fuel injection you could probaly get better fuel management(programer or chip) & gain some HP & torque that would help. If you drop you RPM too much & don't upgrade the power you will lug the engine down & it will be worse. The AT 545 is non lock up, so the only advantage will be at the take off with 1 more gear.
Before I would go to that extend, I would look in repower with a duramax with the allison 1000, then you get torque power & double overdrive with the later model tranny. Even with higher price on fuel, the way to pull weight is still with diesel engine, till I can come out with that water engine.
good luck wrench