We know there is a separate vacuum pump on the engine, thus I'm assuming LR designed the 5.0 to operate with the crankcase under vacuum. I never heard of this before googling around for your issue, but it seems to be a well known engine strategy.
If that assumption is correct, then there must also be a corresponding air inlet to control the amount of vacuum present.
Logically, a clogged inlet would cause excessive vacuum. I'm guessing the rear main is the next easiest point for air to enter the crankcase.
This would also explain your lack of oil leakage. Under vacuum, oil is not going to be leaking past that seal. When vacuum is not present, seal is not deformed and is doing it's job and keeping oil in the case.
I didn't stay at a Holiday Inn last night, but if if my first assumption is correct and the 5.0 is a vacuum case engine, I would be poring over the service manual looking for something labelled crankcase inlet, breather, ventilation, etc, and investigating that.
You guys feel free to shoot holes in my theory, but obviously vacuum is getting to the bellhousing by some route.