Take a peek at you driver side door panel and I'll bet that they have tire pressure there. Normally on a dually 80# is way to much on the rears and the manuf probably calls for somewhere around 60 PSI on the rears and this is for a full loaded truck to the max GVWR.
I agree with using a tire inflation chart for the weight carried but you do need to spend the 9 to $10 to get the weight of your rig and not use someone elses and guess what your camper weighs loaded ready to go camping with everybody and all your stuff in it. Then you well have an idea what pressure to run with the pressure charts.
And there is a difference what people run in a SRW and a DRW so taking what people say when they are running a SRW does not work for you. It would be nice if people that are responding would keep their responses to a DRW as that is what the OP has.
This is IMO