Brake tank pressure builds to 120+. Sometimes when driving i notice it more around 125+, then it settles down.
But I may be able to answer my own mystery. 2 weeks ago I learned that I had a bad leak in the braided air supply line coming out of the compressor on the engine. I'm working to get that replaced, not as easy as I would think. But given the loss rate when the compressor is running, i'm excited to see how much faster it comes up to pressure when I get that leak fixed. I'm not scheduled to go anywhere until December, so I have some time to deal with it. I'm currently in Indio, CA. for the winter. I had a guy in Beumont, CA who was going to fix it, but I've not heard back from him in a week. He was going to have a shop build a custom air line as Prevost seemed reluctant to send him the "only one in the US", located in Tennessee. This is a braided line running from the compressor to an air dryer (?) just above the drive axle. About 15' (?) in length.
Once I get this leak fixed, i definitely want to explore your theories and revisit this.