Tire Pressure?

^What he said.
Without knowing what size tires are on it and how loaded down it is, none of us can say.
The tire kid may be right. And he’s certainly not grossly incorrect for the front axle. Unless you’re somehow loaded @ss heavy, 80psi on pizza cutters up front is ok.
Rear, same thing, although at 12.5k gvwr you’re probably 7500 +\- on the rears. 65 psi would be the minimum except in hot weather/high speed high duty cycle I’d want to be wired up more.
Or if you’re running in real cold weather and/or snow I’d want to be a tad on the low side of “perfect” for traction purposes.
That said 80psi isn’t grossly over pressure in the rears for that rig unless it’s basically empty. And even then it won’t hurt a thing or tire wear. Just ride a little more harsh.

first thing you need to do is weigh the rig ready for the road. Gear on board, fresh water and the like.
Then go to the tire manufacturer website and get the inflation chart.
Inflate the tires according to that.
I run five pounds over chart indicated pressure!

On what was the "young tire tech" basing his recommendations - the max PSI on the side of the tire for load range E tires? The folks who built your RV know more about the requirements than would this young tire tech. Are you seeing any signs of underinflation? If it were me, and in the absence of abnormal wear, I'd stick with the door sticker.

Rob