6.7 Cummins with 68RFE - what gear you tow in?

The truck will pick the right gear for you in normal or tow haul mode but with 6th being a deep overdrive, I'd keep it in 5th gear tops and save the trans a few or a lot of downshifts.
RPMs don't hurt these engines. Lugging the engine is not good for it though and if you drive by the egt you will see how much cooler the post combustion temps are at higher rpms.
Vehicles are made pretty idiot proof but those functions are not always the greatest for longevity.