3.42, wow. Didn't even know they went that low on DRW's. Mine has a 3.73 and often 6th gear (OD) would be too tall if the wind was kicking up too much, so I would run it in 5th (1:1) at about 55 mph. RV trailers are very light and drag heavy, almost like pulling a parachute.
The G56 is probably quieter than the ZF6. ZF6 in Fords came also with DMF which kept them quieter but switch to a regular southbend clutch or whatever and they have a lot of gear rollover noise in lower RPM ranges. The MTL is a 75W80 I believe but I might try the 85 or 90 weight next time. The higher viscosity is a good idea if it is running hard and hot. The other neat thing about the Ford ZF6's is they came with an internal pump and cooling circuit from the factory using a heat exchanger at the bottom of the radiator. The GM application never had that.