I don't have manuals prior to '97, but IIRC (and it has been a LONG time) it was sometime in the early to mid 90s when the electro-hydraulic system came to play. As I recall, the issue was retail motorhome owners were not in the habit of applying the park brake at EVERY stop, prior to placing in park. With the heavy GVWs of motorhomes, if one were to park on a hill, they may not get the unit out of park due to the high loads on the parking pawl in the trans.
So, the auto apply system was born. It was, I believe only on motorhome chassis...until Frito Lay came along. I cannot remember if the park pawl was still in the motorhomes after the auto-apply or if motorhomes ever got the Allison trans, but Frito wanted an Allison (which have no parking pawl) for a 21 year, (3 rebuild lives) duty cycle. That put the auto apply in their commercial chassis. That's where I came into the game. Motorhomes were serviced out of the fleet department, because they were so unique as opposed to retail smaller trucks, though they were handled by a coworker of mine. I just had tribal knowledge passed on by him.
Trivia, for what it's worth.
Lesson here, for you mechanical p-brake owners....always apply the p-brake prior to placing in park. It's a good habit anyway, but it may save you some trouble on a hill sometime, especially if you have a toad behind.
"If the women don't find you handsome, they should at least find you handy." - Red Green