No wheelchair can navigate the corners of the corridor in the Viewliner sleeper or diner cars, unfortunately.
For coach passengers, a narrow wheelchair may be able to run down the aisle and into the dining car. But the other end of the diner (the end next to the sleepers) has sharp turns in the passageway around the kitchen, and the sleeper cars have even sharper turns around the bedrooms.
So if your father is confined to a wheelchair, he (a) needs to reserve the wheelchair-accessible room, and (b) will basically be stuck in his room (it'll have wheelchair access to the car vestibule & door, but that's it). The sleeper attendant is obliged to bring his diner meals (included in the ticket price) to him, and should also be willing to get him stuff from the cafe if he asks.
I notice you say PGH to ORL. If you're doing PGH to WAS, that is a bilevel Superliner and is if anything even less accessible (your father will be on the lower floor, everything else is on the upper floor, even in coach.) If you're doing PGH-PHL, that's all coach (no sleepers) and it may be possible to reserve a space in the coach next to the cafe car.
ORL doesn't have level boarding, PGH doesn't have level boarding, and WAS doesn't have level boarding for Superliners or for trains heading south -- so you'll have to make sure Amtrak gets out the wheelchair lift at any of those stations. I'm not sure if they use wheelchair lifts at Philadelphia, but I think it's level boarding "roll on roll off" there.