70lbs in the rears of a dually unloaded is like driving around w/ roller skates as wheels....

I have 2015 Denali 3500 crew dually, I run 45 psi in my rear tires when not towing and adjust to the load when I do. I dropped the front pressure by 10 lbs of factory spec for a better ride but it still does not come close to the ride my 2001 8.1 Chevy crew dually has.

Truck mfg "recommended" tire inflation is CYA as much as anything. They recommend inflation for full load.
You can definitely air down which will give better tire wear. Running 70+psi unloaded will usually result in greater tire wear in the center of the tread, loaded the tire will flatten out and wear even.
I find 60psi to be about right with my topper but unloaded.
I could not find a full chart of tire size vs pressure vs weight rating. Attached is a Ford truck link example. This is per axle so cut the weight in half per tire.
http://cimg2.ibsrv.net/gimg/www.f150...0e9df0b167.png

Due to government regulations for TPMS parameters that do not consider the differences between a near constant loaded car versus a variable loaded truck, it will alarm higher than comfortable or (prudent for unloaded trucks).
2015 TPMS is much better than the first years of TPMS with my 2008. I have a dash light at 68psi on the rear. About 2010, they lowered it to a more reasonable level.
IF you are wanting TPMS changed, it is possible but finding someone with a Tech 2 that is willing to do it may be a challenge. The value is stored in the BCM (Body Control Module). Instructions can be found searching DieselPlace.com forum.